drm/i915: Fix use of engine->index for register offset

Since commit de1add36 ("drm/i915: Decouple execbuf uAPI from internal
implementation") the index of the engine (its engine->id) in the
internal list no longer matches the hardware id. However, in a couple of
locations we missed fixing up the difference. In this case,
RING_FAULT_REG() refers to engine->id which is now not what the register
offset actually should be. Fortunately, in both case we should be more
or less looping over 0..I915_NUM_ENGINES.

Fixes: de1add36

 ("drm/i915: Decouple execbuf uAPI from internal...")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469643077-2523-2-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: stable@vger.kernel.org