drm/omap: dss: Move display type validation to initialization time

The display type is validated when the display is connected to the DSS
output. We already have all the information we need for validation when
initializing the outputs. Move validation to output initialization to
simplify pipeline connection handling.

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>