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>

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c

@@ -1818,11 +1818,13 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
 	/* Change the size here instead of the init above so only lpfn is affected */
 	amdgpu_ttm_set_buffer_funcs_status(adev, false);
 #ifdef CONFIG_64BIT
+#ifdef CONFIG_X86
 	if (adev->gmc.xgmi.connected_to_cpu)
 		adev->mman.aper_base_kaddr = ioremap_cache(adev->gmc.aper_base,
 				adev->gmc.visible_vram_size);
 
 	else
+#endif
 		adev->mman.aper_base_kaddr = ioremap_wc(adev->gmc.aper_base,
 				adev->gmc.visible_vram_size);
 #endif