drm/amd/display: add missing mask for dcn

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Duke Du <Duke.Du@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>