Currently each drm_fb takes a reference on a client buffer it wraps.
This prevents us from being able to reuse a drm_fb in multiple places
(e.g. two views of the same client buffer) simultaneously, or even back
to back.

Move the buffer reference to the plane state, as preparation for
allowing drm_fb to be cached inside the weston_buffer.

Signed-off-by: Daniel Stone <daniels@collabora.com>