drm/qxl: qxl_release leak in qxl_draw_dirty_fb()

ret should be changed to release allocated struct qxl_release

Cc: stable@vger.kernel.org
Fixes: 8002db63

 ("qxl: convert qxl driver to proper use for reservations")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Link: http://patchwork.freedesktop.org/patch/msgid/22cfd55f-07c8-95d0-a2f7-191b7153c3d4@virtuozzo.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>