gralloc: Let gralloc_priv.h be overloaded by device trees

Adjustments to private_handle_t may be necessary for device-specific
blob compatibility, so don't enforce usage of the local header

Change-Id: I588ca59ed76b1a063a4b663ac2c98589865b7738

Conflicts:
	libgralloc/alloc_controller.cpp
	libgralloc/gpu.h