drm/amd/display: Remove unused defines

[Why]
CONFIG_DRM_AMD_DC_DCN3_0 has been folded into
CONFIG_DRM_AMD_DC_DCN and is not needed.

[How]
Drop CONFIG_DRM_AMD_DC_DCN3_0.

Signed-off-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>