Revert "drm: drop redundant drm_file->is_master"

This reverts commit 48ba813701.

Thanks to Chris:
"drm_file->is_master is not synomous with having drm_file->master ==
drm_file->minor->master. This is because drm_file->master is the same
for all drm_files of the same generation and so when there is a master,
every drm_file believes itself to be the master. Confusion ensues and
things go pear shaped when one file is closed and there is no master
anymore."

Conflicts:
	drivers/gpu/drm/drm_drv.c
	drivers/gpu/drm/drm_stub.c

drivers/gpu/drm/drm_ioctl.c

@@ -607,7 +607,7 @@ static int drm_ioctl_permit(u32 flags, struct drm_file *file_priv)
 		return -EACCES;
 
 	/* MASTER is only for master or control clients */
-	if (unlikely((flags & DRM_MASTER) && !drm_is_master(file_priv) &&
+	if (unlikely((flags & DRM_MASTER) && !file_priv->is_master &&
 		     !drm_is_control_client(file_priv)))
 		return -EACCES;