libhwui: skip sync/flush in FlushLayerUpdates when fence sync is disabled

Change-Id: I9350dbcfc1a7123f176b7b13ecdd34fba4afad04
libhwui: workaround corruption in Broadcom VideoCore GPUs

Change-Id: I4ba2dd7d3ad2183802f7c1d4c117e8a2fe81af38