libgralloc: Allow ionFlags for GRALLOC_USAGE_PRIVATE_MM_HEAP case

Add 'BOARD_CAMERA_USE_MM_HEAP := true' to your device tree to enable

Change-Id: I4596dbb252c07e455edd1ccf2e4f8d233bea5b7c

Conflicts:
	libgralloc/alloc_controller.cpp