drm/radeon: Disable dma rings for bo moves on r6xx

They still seem to cause instability on some r6xx parts.
As a follow up, we can switch to using CP DMA for bo
moves on r6xx as a lighter weight alternative to using
the 3D engine.

A version of this patch should also go to stable kernels.

Tested-by: J.N. <golden.fleeced@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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