drm/omap: Create all planes before CRTCs

Creating all the planes in a single location instead of creating them
per-CRTC with remaining planes then created in a second step simplifies
the logic.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>