drm/amdgpu/display: fix DCN3.2 Makefiles for non-x86

Add proper handling for PPC64.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>