hwc: hwc_sync - populate correct fences for BLIT

Previously, the acquire and release fences for BLIT
composition were populated using same logic as Overlay,
but BLIT composition consists of separate BLIT step
before screen refresh, and app buffers are no longer
required after BLIT. So holding the buffers for both
the steps is unnecessary and impacts performance.

This change adds condition for BLIT in fence population
logic in hwc_sync and populates the right fences for the
App Buffers and Display Refresh during BLIT composition.

Change-Id: I9998a7052c0375acb32a174bdf0469d2a88f79e8