drm/omap: Move common display enable/disable code to encoder

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>