libhwcomposer: support TARGET_NO_HW_VSYNC flag

Change-Id: Id24a2c39bb2520e2af711cb3da35bf9098ba6475
Signed-off-by: Tommy Alex <iptux7@gmail.com>

Conflicts:
	libhwcomposer/hwc.cpp
	libhwcomposer/hwc_vsync.cpp