drm/i915: Capture batchbuffer state upon GPU hang

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: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>