drm/i915: Split up hangcheck phases

In order to simplify hangcheck state keeping, split hangcheck
per engine loop in three phases: state load, action, state save.

Add few more hangcheck actions to separate between seqno, head
and subunit movements. This helps to gather all the hangcheck
actions under a single switch umbrella.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>