drm/amd/display: Check null pointer before try to access it

[why & how]
Make sure plane_state is not null before calling a function
that dereferences it. Besides, remove redundant codes.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>