All .enable() and .disable() handlers for panels and connectors share
common code that validates and updates the device's state. Move it to
common locations in the omap_encoder_enable() and omap_encoder_disable()
handlers.

The enabled check in the .disable() handler is left untouched, it will
be addressed separately.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>