hwc: unset the layer in the vblank_handler

hwc: unset the layer in the vblank_handler

If we unset layer immediately we have flickering because I think it will
be better to unset layer after vblank.

For example the video is being displayed on layer with zpos 0 and UI
is being composited on layer with zpos 1 and HWC decide to composite video
to target_window. Then we do unset for layer 0 and set to layer 1 target buffer
with UI and video and we have the flickering. If we postpone unset till vblank
we don't have the flickering.

Change-Id: I64a5bcb02edb87a82890fc3278a68164c89f71c1
Signed-off-by: Konstantin Drabeniuk <k.drabeniuk@samsung.com>