drm/radeon: Sync ME and PFP after CP semaphore waits v4

Fixes lockups due to CP read GPUVM faults when running piglit on Cape
Verde.

v2 (chk): apply the fix to R600+ as well, on CIK only the GFX CP has
	  a PFP, add more comments to R600 code, enable flushing again
v3: (agd5f): only apply to 7xx+.  r6xx does not have the packet.
v4: (agd5f): split flush change into a separate patch, fix formatting

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>