SurfaceFlinger: Fix surfaceflinger crash when showing screen update

With selecting 'show screen update' option under Setting/Developer
options, there's crash in getYUVBufferCount() in Hwcomposer.cpp.
This occurs because the handles of hwc_layer_list have invalid
values that are passed from HWComposer:createWorkList().
Initialized hwc_layer_list after it is allocated.