[hwc] add new composition type TDM_COMPOSITION_CLIENT_CANDIDATE

If a window with the device composition type changes a type
to the client composition type (by the e20 demands or by
the driver demands) we got the flickering as a buffer owned by
this window can't be composited to the fb_target immediately.

That's why we need the new transition type between the
TDM_COMPOSITION_DEVICE and the TDM_COMPOSITION_CLIENT.

Change-Id: I56c5c20b206c5f8083123c2ca189a0d2e071faf6
Signed-off-by: Sergey Sizonov <s.sizonov@samsung.com>