compositor-drm: Rename drm_sprite to drm_plane

compositor-drm: Rename drm_sprite to drm_plane

We make the differentiation where planes are an abstract framebuffer
with a position within a CRTC/output, and sprites are special cases of
planes that are neither the primary (base/framebuffer) nor cursor plane.

drm_sprite, OTOH, contains nothing that's actually specific to sprites,
and we end up duplicating a lot of code to deal with them, especially
when we come to use an entirely plane-based interface with atomic
modesetting.

Rename drm_sprite to drm_plane, to reflect that it's actually generic.

No functional changes.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
[Pekka: dropped the removal of an unrelated comment]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>