The bbstate contains useful bits of debugging information such as
whether the batch is being read from GTT or PPGTT, or whether it is
allowed to execute privileged instructions.

v2: Only record BB_STATE for gen4+
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>