drm: i2c encoder helper wrappers

Simplify life for drivers using an encoder-slave, so that they can make
their drm_encoder_helper_funcs const, rather than needing to dynamically
allocate and populate them.

Signed-off-by: Rob Clark <robdclark@gmail.com>

diff --cc drivers/gpu/drm/drm_encoder_slave.c
Simple merge

diff --cc include/drm/drm_encoder_slave.h
Simple merge