drm/amdgpu/gfx6: Use PFP on the compute queues too

In the Linux kernel, the following vulnerability has been resolved: drm/amdgpu/gfx6: Use PFP on the compute queues too On GFX6, the compute rings use the same CP path as the graphics ring. The only difference is that they don't support draw commands. (As opposed to GFX7 and newer which have a separate command parser that is called MEC for compute queues.) This means that we have to take into consideration that the PFP also exists on compute queues on GFX6: Use PFP for register writes on both graphics and compute queues. In the pipeline sync, use the PFP to wait for the previous fence (and not the ME) to prevent the PFP from starting to execute the next submission while the ME is still in the previous submission. After a VM flush, emit PFP_SYNC_ME on compute queues as well.