drm/i915: Record the position of the request upon error

So that we can tally the request against the command sequence in the
ringbuffer, or merely jump to the interesting locations.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 5622ec8..b839728 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -183,6 +183,7 @@
 		struct drm_i915_error_request {
 			long jiffies;
 			u32 seqno;
+			u32 tail;
 		} *requests;
 		int num_requests;
 	} ring[I915_NUM_RINGS];