drm/i915: Use correct index for backtracking HUNG semaphores

When decoding the semaphores inside hangcheck, we need to use the hw-id
and not the local array index.

Fixes: de1add3 ("drm/i915: Decouple execbuf uAPI ...")
Testcase: igt/gem_exec_whisper/hang # gen6-7
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161003124516.12388-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_irq.c

@@ -2846,7 +2846,7 @@ semaphore_wait_to_signaller_ring(struct intel_engine_cs *engine, u32 ipehr,
 			if (engine == signaller)
 				continue;
 
-			if (offset == signaller->semaphore.signal_ggtt[engine->id])
+			if (offset == signaller->semaphore.signal_ggtt[engine->hw_id])
 				return signaller;
 		}
 	} else {
@@ -2856,13 +2856,13 @@ semaphore_wait_to_signaller_ring(struct intel_engine_cs *engine, u32 ipehr,
 			if(engine == signaller)
 				continue;
 
-			if (sync_bits == signaller->semaphore.mbox.wait[engine->id])
+			if (sync_bits == signaller->semaphore.mbox.wait[engine->hw_id])
 				return signaller;
 		}
 	}
 
-	DRM_DEBUG_DRIVER("No signaller ring found for ring %i, ipehr 0x%08x, offset 0x%016llx\n",
-			 engine->id, ipehr, offset);
+	DRM_DEBUG_DRIVER("No signaller ring found for %s, ipehr 0x%08x, offset 0x%016llx\n",
+			 engine->name, ipehr, offset);
 
 	return ERR_PTR(-ENODEV);
 }