drm: Update drm_device docs about embedding.

It's supported now! Spotted while reviewing Chris' patch to add a
release hook.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161208102847.3063-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_drv.c

@@ -323,9 +323,8 @@ void drm_minor_release(struct drm_minor *minor)
  * historical baggage. Hence use the reference counting provided by
  * drm_dev_ref() and drm_dev_unref() only carefully.
  *
- * Also note that embedding of &drm_device is currently not (yet) supported (but
- * it would be easy to add). Drivers can store driver-private data in the
- * dev_priv field of &drm_device.
+ * It is recommended that drivers embed struct &drm_device into their own device
+ * structure, which is supported through drm_dev_init().
  */
 
 /**
@@ -462,7 +461,11 @@ static void drm_fs_inode_free(struct inode *inode)
  * Note that for purely virtual devices @parent can be NULL.
  *
  * Drivers that do not want to allocate their own device struct
- * embedding struct &drm_device can call drm_dev_alloc() instead.
+ * embedding struct &drm_device can call drm_dev_alloc() instead. For drivers
+ * that do embed struct &drm_device it must be placed first in the overall
+ * structure, and the overall structure must be allocated using kmalloc(): The
+ * drm core's release function unconditionally calls kfree() on the @dev pointer
+ * when the final reference is released.
  *
  * RETURNS:
  * 0 on success, or error code on failure.