drm/omap: Pass pipe pointer to omap_crtc_init()

Replace the dss display device pointer by a pipe pointer that will allow
the omap_crtc_init() function to access both the display and the DSS
output. As a result we can remove the omapdss_device_get_dispc_channel()
function that is now unneeded.

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>