hwc: Unlock prev. buffers in N+1th draw round.

Unlock previously locked overlay buffers in N+1th draw round, rather than N+2th.
This is now possible because of wait for PAN.
Provides an opportunity to improve genlock behavior.

Signed-off-by: Saurabh Shah <saurshah@codeaurora.org>

Change-Id: I13d9c39e276918c3d3d27b2f07ed486b27b58441
1 file changed