This gives BOs which haven't been accessed by the CPU since they were
moved to visible VRAM another chance to stay in VRAM when another BO
needs to go to visible VRAM.

This should allow BOs to stay in VRAM longer in some cases.

v2:
* Only do this for BOs which don't have the
  AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED flag set.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>