Change the type of cursor_plane from a weston_plane (base tracking
structure) to a drm_plane (wrapper containing additional DRM-specific
details), and make it a dynamically-allocated pointer.

Using the standard drm_plane allows us to reuse code which already deals
with drm_planes, e.g. a common cleanup function.

This patch introduces a 'special plane' helper, creating a drm_plane
either from a real KMS plane when using universal planes, or a fake plane
otherwise. Without universal planes, the cursor and primary planes are
hidden from us; this helper allows us to pretend otherwise.

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