hwc: Set DMA mode to LINE after UNSET, commit of BLOCK session

Set DMA mode to LINE only after we UNSET and commit the sessions
in BLOCK mode from previous rounds.
Currently, it is incorrectly set to LINE always at the beginning
of a draw cycle.
Now we set it to LINE only at the end of a draw cycle if no rotator
sessions are active.

Change-Id: I6e2351d214d01764eced0673b639f709fbd193c3
2 files changed