drm/radeon: also try GART for CPU accessed buffers

commit 544092596e8ac269f70e70961b5e9381909c9b1e upstream.

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>
[bwh: Backported to 3.2: ttm_bo_validate() takes an extra no_wait_reserve
 parameter; keep passing true]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>