drm/i915: Fix up some stray to_i915(dev) after a recent merge

The merge conflict resolution caused some dev->dev_private to return
from the dead. Kill them with to_i915().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469644349-24571-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_sysfs.c

@@ -323,7 +323,7 @@ static ssize_t gt_boost_freq_mhz_store(struct device *kdev,
 {
 	struct drm_minor *minor = dev_to_drm_minor(kdev);
 	struct drm_device *dev = minor->dev;
-	struct drm_i915_private *dev_priv = dev->dev_private;
+	struct drm_i915_private *dev_priv = to_i915(dev);
 	u32 val;
 	ssize_t ret;