drm/amdgpu/display: fix CFLAGS setup for DCN30

Properly handle clang and older versions of gcc.

Fixes: e77165bf ("drm/amd/display: Add DCN3 blocks to Makefile")
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>