hwc: Do not use copybit if planeAplha and downscaling invloved

C2D can not handle plane alpha with downscaling properly, so
do not use copybit for such layers.

Conflicts:
        libhwcomposer/hwc_copybit.cpp

CRs-Fixed: 617718
Change-Id: I879bec6365a09bcaa803ed1a765f1e4dd9999474