drm/omap: Group CRTC, encoder, connector and dssdev in a structure

Create an omap_drm_pipeline structure to model display pipelines, made
of a CRTC, an encoder, a connector and a DSS display device. This allows
grouping related parameters together instead of storing them in
independent arrays and thus improves code readability.

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>