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