We may keep the crtc's enabled when userspace unsets all framebuffers but
keeps the crtc active. This exposes a WARN in fbc_global disable, and
a lot of bugs in our hardware readout code. Solve this by disabling
all crtc's for now.

Changes since v1:
- Use lock_all_ctx instead of lock_all.
Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481812185-19098-4-git-send-email-maarten.lankhorst@linux.intel.comReviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>