drm: atmel-hlcdc: add discard area support

The HLCDC IP provides a way to discard a specific area on the primary
plane (in case at least one of the overlay is activated and alpha
blending is disabled).
Doing this will reduce the amount of data to transfer from the main
memory to the Display Controller, and thus alleviate the load on the
memory bus (since this link is quite limited on such hardware,
this kind of optimization is really important).

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>