drm/amd/display: Return UPDATE_TYPE_FULL on writeback update

Should do full update when display writeback is updated.

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Duke Du <Duke.Du@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>