drm/amd/display: Add missing reg mask for dcn3

This field is missing for dcn3 so add it from dcn20.

enc2_set_dynamic_metadata() trys to sets this and we get a
generic_reg warning since the mask is not defined.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>