drm/amdgpu/display: fold DRM_AMD_DC_DCN3_1 into DRM_AMD_DC_DCN

No need for a separate flag now that DCN3.1 is not in bring up.
Fold into DRM_AMD_DC_DCN like previous DCN IPs.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>