drm/tilcdc: Remove the use of drm_device private_data

drm/tilcdc: Remove the use of drm_device private_data

The DRM core documentation recommends against using dev_private:
"Instead of using this pointer it is recommended that drivers use
embed the struct &drm_device in their larger per-device structure."

This patch refactors the tilcdc driver to follow this recommendation
by embedding struct drm_device within struct tilcdc_drm_private and
replacing all dev->dev_private accesses with the ddev_to_tilcdc_priv()
helper macro that uses container_of().

This change aligns the driver with modern DRM best practices.

Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Kory Maincent (TI.com) <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20260123-feature_tilcdc-v5-17-5a44d2aa3f6f@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>