Add cma Fbdev, Fbdev is legency and optional, you can enable/disable it by
configuring DRM_FBDEV_EMULATION.
Add hotplug.

v8: None.
v7: None.
v6: None.
v5: None.
v4: None.
v3: None.
v2:
- Use CONFIG_DRM_FBDEV_EMULATION instead of CONFIG_DRM_HISI_FBDEV.
Signed-off-by: NXinliang Liu <xinliang.liu@linaro.org>
Signed-off-by: NXinwei Kong <kong.kongxinwei@hisilicon.com>