drm/i2c: tda998x: allow re-use of tda998x support code

Re-jig the TDA998x code so that we separate the functionality from the
drm slave encoder implementation.  In several places, this is pretty
clearly the correct thing to do, because we can avoid repetitively
having to convert from the drm_encoder to the TDA998x private
structure, particularly with the driver internal functions.

The main motivation behind this change is to allow the code to be
re-used with a standard drm_encoder and drm_connector implementation
based on the component helpers, rather than the slave_encoder system.
The addition of this will be in the following patch.

We keep the slave_encoder interface as there are existing users of
this; we need to give them time to convert and test.

Tested-by: Darren Etheridge <detheridge@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>