drm/i2c: tda998x: add component support

Add component helper support to the tda998x driver.  This permits the
TDA998x to be declared as a separate device in device tree, and bound
at the appropriate moment with a co-operating card driver.

The existing slave_encoder interfaces are kept while there are existing
users of it in order to prevent regressions.

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