drm: check that ->set_config properly updates the fb

Historically drm lacked fb refcounting, so the updating of crtc->fb
was done by the lower levels at a point convenient to get their own
refcounting (e.g. refcounts for the underlying gem bo, pinning
refcounts) right. With the introduction of refcounted fbs the drm core
handled the fb refcounts, but still relied on drivers to update the
crtc->fb pointer (this approach required the least invasive changes in
drivers).

Enforce this contract with a WARN_ON.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>