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