Document structs drm_encoder_slave_funcs, drm_encoder_slave, and
drm_i2c_encoder_driver.

v2: Actually document the structs instead of just silencing kernel-doc
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/19bc9672c8ae4f7aee235665a4d2360e8790193d.1709898638.git.jani.nikula@intel.com