drm/radeon: switch back to the CP ring for VM PT updates

For large VM page table updates, we can sometimes generate
more packets than there is space on the ring.  This happens
more readily with the DMA ring since it is 64K (vs 1M for the
CP).  For now, switch back to the CP.  For the next kernel,
I have a patch to utilize IBs for VM PT updates which
alleviates this problem.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=58354

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>