drm: qxl: add missing return check

My static checker complains that "release" is uninitialized if
qxl_alloc_release_reserved() fails, so let's add a check for that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170314075410.GB5984@mwanda


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>