drm/tilcdc: Add support for external tda998x encoder

Add support for an external compontised DRM encoder. The external
encoder can be connected to tilcdc trough device tree graph binding.
The binding document for tilcdc has been updated. The current
implementation supports only tda998x encoder.

To be able to filter out the unsupported video modes the tilcdc driver
needs to hijack the external connectors helper functions. The tilcdc
installes new helper functions that are otherwise identical to
orignals, but the mode_valid() call-back check the mode first localy,
before calling the original call-back. The tilcdc dirver restores the
original helper functions before it is unbound from the external
device.

I got the idea and some lines of code from Jean-Francois Moine's
"drm/tilcdc: Change the interface with the tda998x driver"-patch.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>