compositor-drm: Drop output from release_fb

compositor-drm: Drop output from release_fb

We only need it for the GBM surface the FB was originally created
against; a mismatch here is very bad indeed, so no reason to pass it in
explictly every time rather than store it.

Following patches change drm_fb to be explicitly reference counted; in
order to reduce churn, rename drm_output_release_fb to drm_fb_unref
whilst changing its call signature here, even though it does not yet
actually perform reference counting.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>