drm/omap: dss: Move platform_device_register from core.c to dss.c probe

Register the omapdrm device when we know that dss device probe going
to succeed. This avoids DSS6 and DSS2 omapdrm device registration from
colliding with each other.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
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>