[hwc] add the ability to ask a client(E20) about the revalidation

[hwc] add the ability to ask a client(E20) about the revalidation

The backends may need to ask E20 for revalidation
for some reason (power consumption, performance,...).

So we need some way to let backend do this.

The new TDM API function:
 tdm_output_set_need_validate_handler();

and TDM/backend interface API function:
 tdm_backend_trigger_need_validate_event();

have been added to let backend ask client(E20) for the
revalidation.

Change-Id: Ife266a55043cbf420702098b06223e46ef67c041
Signed-off-by: Sergey Sizonov <s.sizonov@samsung.com>