compositor-drm: Use drm_fb for cursor buffers

compositor-drm: Use drm_fb for cursor buffers

Now that we have better types in drm_fb, use it for cursor buffers as
well. This gives us easier refcounting for our cursors, as well as a
unified buffer-destruction path.

Currently this makes no difference, as the KMS legacy cursor update API
uses GEM names directly, and never touches DRM FBs. However, the cursor
plane becomes a regular KMS plane under atomic, at which point we
require DRM FBs.

Take the opportunity to move to drm_fb ahead of time.

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