drm/qxl: fix memory leak in release list handling

wow no idea how I got this far without seeing this,
leaking the entries in the list makes kmalloc-64 slab grow.

References: https://bugzilla.kernel.org/show_bug.cgi?id=65121


Cc: stable@vger.kernel.org
Reported-by: Matthew Stapleton <matthew4196@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>