drm/i915: Introduce i915_gem_active for request tracking

In the next patch, request tracking is made more generic and for that we
need a new expanded struct and to separate out the logic changes from
the mechanical churn, we split out the structure renaming into this
patch.

v2: Writer's block. Add some spiel about why we track requests.
v3: Now i915_gem_active.
v4: Now with i915_gem_active_set() for attaching to the active request.
v5: Use i915_gem_active_set() from inside the retirement handlers

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470293567-10811-10-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index a6bab90..17a206f 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2242,11 +2242,10 @@
 	 * requests on one ring where the write request is older than the
 	 * read request. This allows for the CPU to read from an active
 	 * buffer by only waiting for the write to complete.
-	 * */
-	struct drm_i915_gem_request *last_read_req[I915_NUM_ENGINES];
-	struct drm_i915_gem_request *last_write_req;
-	/** Breadcrumb of last fenced GPU access to the buffer. */
-	struct drm_i915_gem_request *last_fenced_req;
+	 */
+	struct i915_gem_active last_read[I915_NUM_ENGINES];
+	struct i915_gem_active last_write;
+	struct i915_gem_active last_fence;
 
 	/** Current tiling stride for the object, if it's tiled. */
 	uint32_t stride;