fb: handle allocation failure in alloc_apertures()

If the kzalloc() fails we should return NULL.  All the places that call
alloc_apertures() check for this already.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: James Simmons <jsimmons@infradead.org>
Acked-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>