qxl: add ring prep code for s/r

This prepare the ring code for s/r additions, the release ring will need
reinitialising.

Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/qxl/qxl_cmd.c
Simple merge

diff --cc drivers/gpu/drm/qxl/qxl_drv.h
Simple merge