drm/i915: Use the engine name directly in the error_state file

Instead of using local string names that we will have to keep
maintaining, use the engine->name directly.

v2: Better invalid engine_id handling, capture_bo will not be able know
the engine_id and end up with -1 (Michal).

Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180110012151.28261-1-michel.thierry@intel.com


[ickle: minor massaging of function names]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180118175228.2830-1-chris@chris-wilson.co.uk