drm/amd/display: Check for NULL pointer

drm/amd/display: Check for NULL pointer

[why & how]
Need to make sure plane_state is initialized
before accessing its members.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Xi (Alex) Liu <xi.liu@amd.com>
Signed-off-by: Sung Joon Kim <sungjoon.kim@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>