get_modes() callback might be called asynchronously from the DRM core and
it is not synchronized with bridge_enable(), which sets proper runtime PM
state of the main DP device. Fix this by calling pm_runtime_get_sync()
before calling drm_get_edid(), which in turn calls drm_dp_i2c_xfer() and
analogix_dp_transfer() to ensure that main DP device is runtime active
when doing any access to its registers.

This fixes the following kernel issue on Samsung Exynos5250 Snow board:
Unhandled fault: imprecise external abort (0x406) at 0x00000000
pgd = c0004000
[00000000] *pgd=00000000
Internal error: : 406 [#1] PREEMPT SMP ARM
Modules linked in:
CPU: 0 PID: 62 Comm: kworker/0:2 Not tainted 4.13.0-rc2-00364-g4a97a3da #3357
Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
Workqueue: events output_poll_execute
task: edc14800 task.stack: edcb2000
PC is at analogix_dp_transfer+0x15c/0x2fc
LR is at analogix_dp_transfer+0x134/0x2fc
pc : [<c0468538>]    lr : [<c0468510>]    psr: 60000013
sp : edcb3be8  ip : 0000002a  fp : 00000001
r10: 00000000  r9 : edcb3cd8  r8 : edcb3c40
r7 : 00000000  r6 : edd3b380  r5 : edd3b010  r4 : 00000064
r3 : 00000000  r2 : f0ad3000  r1 : edcb3c40  r0 : edd3b010
Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
Control: 10c5387d  Table: 4000406a  DAC: 00000051
Process kworker/0:2 (pid: 62, stack limit = 0xedcb2210)
Stack: (0xedcb3be8 to 0xedcb4000)
[<c0468538>] (analogix_dp_transfer) from [<c0424ba4>] (drm_dp_i2c_do_msg+0x8c/0x2b4)
[<c0424ba4>] (drm_dp_i2c_do_msg) from [<c0424e64>] (drm_dp_i2c_xfer+0x98/0x214)
[<c0424e64>] (drm_dp_i2c_xfer) from [<c057b2d8>] (__i2c_transfer+0x140/0x29c)
[<c057b2d8>] (__i2c_transfer) from [<c057b4a4>] (i2c_transfer+0x70/0xe4)
[<c057b4a4>] (i2c_transfer) from [<c0441de4>] (drm_do_probe_ddc_edid+0xb4/0x114)
[<c0441de4>] (drm_do_probe_ddc_edid) from [<c0441e5c>] (drm_probe_ddc+0x18/0x28)
[<c0441e5c>] (drm_probe_ddc) from [<c0445728>] (drm_get_edid+0x124/0x2d4)
[<c0445728>] (drm_get_edid) from [<c0465ea0>] (analogix_dp_get_modes+0x90/0x114)
[<c0465ea0>] (analogix_dp_get_modes) from [<c0425e8c>] (drm_helper_probe_single_connector_modes+0x198/0x68c)
[<c0425e8c>] (drm_helper_probe_single_connector_modes) from [<c04325d4>] (drm_setup_crtcs+0x1b4/0xd18)
[<c04325d4>] (drm_setup_crtcs) from [<c04344a8>] (drm_fb_helper_hotplug_event+0x94/0xd0)
[<c04344a8>] (drm_fb_helper_hotplug_event) from [<c0425a50>] (drm_kms_helper_hotplug_event+0x24/0x28)
[<c0425a50>] (drm_kms_helper_hotplug_event) from [<c04263ec>] (output_poll_execute+0x6c/0x174)
[<c04263ec>] (output_poll_execute) from [<c0136f18>] (process_one_work+0x188/0x3fc)
[<c0136f18>] (process_one_work) from [<c01371f4>] (worker_thread+0x30/0x4b8)
[<c01371f4>] (worker_thread) from [<c013daf8>] (kthread+0x128/0x164)
[<c013daf8>] (kthread) from [<c0108510>] (ret_from_fork+0x14/0x24)
Code: 0a000002 ea000009 e2544001 0a00004a (e59537c8)
---[ end trace cddc7919c79f7878 ]---
Reported-by: NMisha Komarovskiy <zombah@gmail.com>
CC: stable@vger.kernel.org # v4.10+
Signed-off-by: NMarek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: NArchit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171121074936.22520-1-m.szyprowski@samsung.com