hwc: [8x26]Never allow primary get DMA pipe when external connect

For 8x26, if external display is connected, DMA pipe should never
be occupied by primary display. Otherwise, external display would
lack DMA pipe if switch to GPU composition.

This patch refines former commit: 9c1eb2be21a4952ee097d33

CRs-Fixed: 640934
Change-Id: Idc64b724cfacf9e8e22ca73e4e443e8d989e1212