drm: sti: add TVOut driver

TVout hardware block is responsible to dispatch the data flow coming
from compositor block to any of the output (HDMI or Analog TV).
It control when output are start/stop and configure according the
require flow path.

TVout is the parent of HDMI and HDA drivers and bind them at runtime.

Tvout is mapped on drm_encoder structure.
One encoder is created for each of the sub-devices and link to their
connector/bridge

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>