drm/tilcdc: Rename external_encoder and external_connector to encoder and connector

drm/tilcdc: Rename external_encoder and external_connector to encoder and connector

Remove the "external_" prefix from encoder and connector members in the
tilcdc driver. These are internal driver structures and the "external"
naming is misleading. The simpler names better reflect that these are
the primary encoder and connector managed by this driver.

Also rename tilcdc_attach_external_device() to tilcdc_encoder_create()
for consistency and to better describe the function's purpose.

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-v5-11-5a44d2aa3f6f@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>