drm/radeon: use gart for DMA IB tests

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

Port of adfed2b0587289013f8143c54913ddfd44ac1fd3
(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

diff --cc drivers/gpu/drm/radeon/cik_sdma.c
Simple merge

diff --cc drivers/gpu/drm/radeon/r600_dma.c
Simple merge