drm/amdgpu/dc: Really fix DCN3.1 Makefile for PPC64

Also copy over the part that makes old gcc handling cross-platform.

Fixes: df7a1658 ("drm/amdgpu/dc: fix DCN3.1 Makefile for PPC64")
Fixes: 926d6972 ("drm/amd/display: Add DCN3.1 blocks to the DC Makefile")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>