drm: Document device unplug infrastructure

While at it, also ocd and give them a consistent drm_dev_ prefix, like
the other device instance functionality. Plus move the functions into
the right places.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170802115604.12734-3-daniel.vetter@ffwll.ch
diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index 59b75a9..b3c6e99 100644
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -436,7 +436,7 @@ int drm_release(struct inode *inode, struct file *filp)
 
 	if (!--dev->open_count) {
 		drm_lastclose(dev);
-		if (drm_device_is_unplugged(dev))
+		if (drm_dev_is_unplugged(dev))
 			drm_put_dev(dev);
 	}
 	mutex_unlock(&drm_global_mutex);