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.

Change-Id: I2b2c3e17f254e3c5e3706bd43de8a244d22a9bb6
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>

Conflicts:
	services/surfaceflinger/DisplayHardware/HWComposer.cpp