Generic Display Pipeline are one of the compositor input sub-devices.
GDP are dedicated to graphic input like RGB plans.
GDP is part of Compositor hardware block which will be introduce later.

A sti_layer structure is used to abstract GDP calls from Compositor.
Signed-off-by: NBenjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: NRob Clark <robdclark@gmail.com>