drm/i2c: adv7511: Refactor encoder slave functions

ADV7511 is represented as an i2c drm slave encoder device. ADV7533, on
the other hand, is going be a normal i2c client device creating bridge
and connector entities.

Move the code in encoder slave functions to generate helpers that are
agnostic to the drm object type. These helpers will later also be used
by bridge and connecter helper functions.

Signed-off-by: Archit Taneja <architt@codeaurora.org>