drm/radeon: set VM base addr using the PFP v2

Seems to make VM flushes more stable on SI and CIK.

v2: only use the PFP on the GFX ring on CIK

Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>