commit 0b021c58 upstream.

Use gart rather than vram to avoid having to deal with
the HDP cache.

Port of adfed2b0
(drm/radeon: use gart memory for DMA ring tests)
to the IB tests.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[ kamal: backport to 3.13-stable: context ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>