drm/radeon/kms/evergreen: fix gpu hangs in userspace accel code

These VGT regs need to be programmed via the ring rather than
MMIO as on previous asics (r6xx/r7xx).

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>