drm/amdgpu: use top down allocation for non-CPU accessible vram

Should help avoid fragmentation of vram due to CPU access
requirements.

Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 			placements[c].fpfn =
 				adev->mc.visible_vram_size >> PAGE_SHIFT;
 			placements[c++].flags = TTM_PL_FLAG_WC | TTM_PL_FLAG_UNCACHED |
-				TTM_PL_FLAG_VRAM;
+				TTM_PL_FLAG_VRAM | TTM_PL_FLAG_TOPDOWN;
 		}
 		placements[c].fpfn = 0;
 		placements[c++].flags = TTM_PL_FLAG_WC | TTM_PL_FLAG_UNCACHED |