drm/omap: Store CRTC lookup by channel table in omap_drm_private

The omap_crtcs global array is used to store pointers to omap_crtc
indexed by DISPC channel number, in order to look them up in the dss_mgr
operations. Store the information in the omap_drm_private structure in
the form of an array of omap_drm_pipeline pointers.

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>