drm/i2c: tda998x: find the drm_device via the drm_connector

This prepares for being a drm_bridge which will not register the
encoder. That makes the connector the better choice.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>