gralloc: Fixes for software composition

Check that the buffer is actually an ion buffer
before calling clean. This is needed for the case
where the physically contiguous framebuffer is mapped.
That framebuffer memory is used for bringup.

Change-Id: I2db84d4bfc3465d995f12e0860be3cafa0d4a81b

Conflicts:
	msm8974/libgralloc/mapper.cpp