drm/radeon: use gart memory for DMA ring tests

Avoids HDP cache flush issues when using vram which can
cause ring test failures on certain boards.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Alexander Fyodorov <halcy@yandex.ru>
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

diff --cc drivers/gpu/drm/radeon/radeon.h
Simple merge