drm/amdgpu: put gtt at 0 in the internal address space

There still seem to be some blocks that make accesses
in the lower part of the address space. This works around
this.

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

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

···
 			dev_warn(adev->dev, "limiting GTT\n");
 			mc->gtt_size = size_bf;
 		}
-		mc->gtt_start = (mc->vram_start & ~mc->gtt_base_align) - mc->gtt_size;
+		mc->gtt_start = 0;
 	} else {
 		if (mc->gtt_size > size_af) {
 			dev_warn(adev->dev, "limiting GTT\n");