drm: sti: add VID layer

VIDeo plug are one of the compositor input sub-devices.
VID are dedicated to video inputs like YUV plans.

Like GDP, VID are part of Compositor hardware block
and use sti_layer structure to provide an abstraction for
Compositor calls.

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