sf: Add a NULL check in getDisplayConfigs

Validate the display binder by adding a NULL check in getDisplayConfigs.
This will prevent a false match if the caller queries the display
configs for an inactive display (whose binder is NULL by default).

Without this change we might end up attempting to index the display
config array, which is unpopulated for inactive displays, and this will
result in a crash. (See getDisplayInfo in SurfaceComposerClient.cpp for
an example of this scenario)

Change-Id: I1a12f43b7c375b9c01998dadd5b658275c733fb2