drm/nouveau: vendor in drm_encoder_slave API

Nouveau driver is the only user of the drm_encoder_slave API. Rework
necessary bits of drm_encoder_slave into the nouveau_i2c_encoder API and
drop drm_encoder_slave.c from the DRM KMS helper.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250106-nouveau-encoder-slave-v3-2-1d37d2f2c67f@linaro.org