drm/qxl: use ttm bo priorities

Allow to set priorities for buffer objects.  Use priority 1 for surface
and cursor command releases.  Use priority 0 for drawing command
releases.  That way the short-living drawing commands are first in line
when it comes to eviction, making it *much* less likely that
ttm_bo_mem_force_space() picks something which can't be evicted and
throws an error after waiting a while without success.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210217123213.2199186-4-kraxel@redhat.com