drm/omap: introduce omap_hw_overlay

Split out the hardware overlay specifics from omap_plane.
To start, the hw overlays are statically assigned to planes.

The goal is to eventually assign hw overlays dynamically to planes
during plane->atomic_check() based on requested caps (scaling, YUV,
etc). And then perform hw overlay re-assignment if required.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211117141928.771082-5-narmstrong@baylibre.com