drm/tilcdc: Rename tilcdc_external to tilcdc_encoder

drm/tilcdc: Rename tilcdc_external to tilcdc_encoder

The tilcdc_external module describes the encoder part of the tilcdc
driver. Rename it to tilcdc_encoder for better clarity and to make
the naming more consistent with DRM subsystem conventions, where
encoder-related files typically use "encoder" in their names.

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-v5-12-5a44d2aa3f6f@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>