drm/amd/display: add NULL check to avoid kernel crash in DC.

drm/amd/display: add NULL check to avoid kernel crash in DC.

[why]
There is case when the userspace like IGT test updates the custom
timings, i.e. the number of active lines CRTC property value,
and without committing the change, followed by
resetting the display, creating FB and plane to the pipe
and committing. The NULL pointer of pipe_ctx->plane_state
occurs and result in kernel crash. We need to avoid that.

[how]
add pointer check for the dc_plane_state of the pipe context in
the call of committing planes for stream in DC component.

Signed-off-by: Robin Singh <robin.singh@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>