drm/vc4: Fix OOPSes from trying to cache a partially constructed BO.

commit ca39b449 upstream.

If a CMA allocation failed, the partially constructed BO would be
unreferenced through the normal path, and we might choose to put it in
the BO cache.  If we then reused it before it expired from the cache,
the kernel would OOPS.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: c826a6e1 ("drm/vc4: Add a BO cache.")
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170301185602.6873-2-eric@anholt.net


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>