compositor-drm: Rename fb_plane to scanout_plane

All planes being displayed have a framebuffer. What makes 'fb_plane'
special is that it's being displayed as the primary plane by KMS.

Previous patchsets renamed this to 'primary_plane' to match the KMS
terminology, namely the CRTC's base plane, which is controlled by
drmModeSetCrtc in the legacy API, and identified by PLANE_TYPE ==
"Primary" in the universal-plane API.

However, Weston uses 'primary_plane' internally to refer to the case
where client content is _not_ directly displayed on a plane, but
composited via the renderer, with the result of the compositing then
shown.

Rename to 'scanout_plane' as our least-ambiguous name, and document it a
bit.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emre Ucan <eucan@de.adit-jv.com>