hwc: Fix Line to Block switch on primary for targets with DMA Multiplexing

DMA multiplexing is supported on WB mode only.
For primary the DMA multiplexing should be avoided
as it can lead to hung rotator.
This can happen when the DMA is being unset from overlay
while at the same time rotator gets configured.

Change-Id: I0c2cb8a5a1ebad8a8e4ed8107d5585836bc42451
Merge pull request #11 from CyanogenMod/cm-11.0

hwc: Fix Line to Block switch on primary for targets with DMA Multiplexi...