drm/i915: Use shorter route to dev_private where possible

Where we have a request we can use req->i915 directly instead
of going through the engine and device. Coccinelle script:

@@
function f;
identifier r;
@@
f(..., struct drm_i915_gem_request *r, ...)
{
...
- engine->dev->dev_private
+ r->i915
...
}
@@
struct drm_i915_gem_request *req;
@@
(
  req->
- engine->dev->dev_private
+ i915
)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1458219850-21007-1-git-send-email-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -2499,7 +2499,7 @@ int intel_ring_begin(struct drm_i915_gem_request *req,
 
 	WARN_ON(req == NULL);
 	engine = req->engine;
-	dev_priv = engine->dev->dev_private;
+	dev_priv = req->i915;
 
 	ret = i915_gem_check_wedge(&dev_priv->gpu_error,
 				   dev_priv->mm.interruptible);