drm/radeon: use gart for DMA IB tests

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>
Cc: stable@vger.kernel.org