drm/omap: Add ovl checking funcs to dispc_ops

In order to be able to dynamically assign overlays to planes we need to
be able to asses the overlay capabilities.

Add a helper function to be able to retrieve the supported capabilities
of an overlay.

And export the function to check if a fourcc is supported on a given
overlay.

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/20211117141928.771082-4-narmstrong@baylibre.com