drm/i915: Add support for GPU reset on gen6.

This has proven sufficient to recover from a hang of the GPU using the
gem_bad_blit test while at the KMS console then starting X.  When
attempting the same during an X session, the timer doesn't appear to
trigger.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>