drm/qxl: avoid dependency lock

qxl_bo_unref calls drm_gem_object_unreference_unlocked which
locks dev->struct_mutex. However this lock could be already
locked if the call came from qxl_gem_object_free.
As we don't need to call qxl_bo_ref/qxl_bo_unref cause
qxl_release_list_add will hold a reference by itself avoid
to call them and the possible deadlock.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>