drm: Propagate error code from fb_create()

Change the interface to expect a PTR_ERR specifing the real error code
as opposed to assuming a NULL return => -EINVAL. Just once the user may
not be at fault!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>