qxl: keep separate release_bo pointer

qxl expects that list_first_entry(release->bos) returns the first
element qxl added to the list.  ttm_eu_reserve_buffers() may reorder
the list though.

Add a release_bo field to struct qxl_release and use that instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20180418054257.15388-3-kraxel@redhat.com


Signed-off-by: Sean Paul <seanpaul@chromium.org>