hwc: Fix operator logic on acquiring fences count

Without this, the screen tears a lot, because overlay based composition are not allowed to get the fences count, so MSMFB_BUFFER_SYNC ioctl will fail.

Change-Id: I24e6b325762f70623af5d83c76540d43c3de2f49

Conflicts:

	libhwcomposer/hwc_utils.cpp