surfaceflinger: Don't crash on missing functions in hwcomposers

Some hwcomposers -- most notably hwcomposer.default.so -- don't
implement blank() or eventControl().
Don't crash when trying to use them.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>

Conflicts:
	services/surfaceflinger/DisplayHardware/HWComposer.cpp

Change-Id: I885bd7e60cf8ea29cf8e01c8eb800c71dbe888ee