drm/omap: Don't store display pointer in omap_connector structure

Display pipelines based on drm_bridge are handled from the bridge
closest to the CRTC. To move to that model we thus need to transition
away from walking pipelines in the other direction, and from accessing
the device at the end of the pipeline when possible.

Remove most accesses to the display device from the omap_connector
implementation, and don't store it in the omap_connector structure.

- For debug messages we can simply use the connector name instead.
- For type checks we can use the drm_connector type.
- For operation lookup we can start at the other end of the pipeline and
  locate the last matching device.

The display device is still passed to the connector init function in
order to find its type, which requires access to the end of the
pipeline.

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>