drm/vc4: Notify the firmware when DRM is in charge

Once the call to drm_fb_helper_remove_conflicting_framebuffers() has
been made, simplefb has been unregistered and the KMS driver is entirely
in charge of the display.

Thus, we can notify the firmware it can free whatever resource it was
using to maintain simplefb functional.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>