drm/amdgpu: fold CONFIG_DRM_AMD_DC_DCN3* into CONFIG_DRM_AMD_DC_DCN (v3)

Avoids confusion in configurations.

v2: fix build when CONFIG_DRM_AMD_DC_DCN is disabled
v3: rebase on latest code

Reviewed-by: Luben Tuikov <luben.tuikov@amd.com> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>