drm/amdgpu: fix compile error on architecture s390 (v2)

ioremap_cache is not supported on some architecture
such as s390. Put the codes into a #ifdef to fix
some compile error reported by test robot.

v2: squash in non-x86 fix

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reported-by: Kernel test robot <lkp@intel.com>
Reviewed-by: Christian Konig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>