drm/amd/display: don't enable DRM CRTC degamma property for DCE

DM maps DRM CRTC degamma to DPP (pre-blending) degamma block, but DCE doesn't
support programmable degamma curve anywhere. Currently, a custom degamma is
accepted by DM but just ignored by DCE driver and degamma correction isn't
actually applied. There is no way to map custom degamma in DCE, therefore, DRM
CRTC degamma property shouldn't be enabled for DCE drivers.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>