drm/radeon: also try GART for CPU accessed buffers

Placing them exclusively into VRAM might not work all the time.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=78297



Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org