drm/omap: dss: Split omapdss_register_display()

Split the function into omapdss_display_init() to perform
display-specific initialization of the omap_dss_device, and
omapdss_register_display() to register the device. The latter will then
be replaced by more generic registration.

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>