drm/amd/display: Remove redundant member from amdgpu_plane.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>