drm/i915: Rearrange the comments in i915_add_request()

The comments were a little out-of-sequence with the code, forcing the
reader to jump around whilst reading. Whilst moving the comments around,
add one to explain the context reference.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2062,16 +2062,19 @@ int __i915_add_request(struct intel_ring_buffer *ring,
 	request->ring = ring;
 	request->head = request_start;
 	request->tail = request_ring_position;
-	request->ctx = ring->last_context;
-	request->batch_obj = obj;
 
 	/* Whilst this request exists, batch_obj will be on the
 	 * active_list, and so will hold the active reference. Only when this
 	 * request is retired will the the batch_obj be moved onto the
 	 * inactive_list and lose its active reference. Hence we do not need
 	 * to explicitly hold another reference here.
 	 */
+	request->batch_obj = obj;
 
+	/* Hold a reference to the current context so that we can inspect
+	 * it later in case a hangcheck error event fires.
+	 */
+	request->ctx = ring->last_context;
 	if (request->ctx)
 		i915_gem_context_reference(request->ctx);