drm: Extract drm_is_current_master

Just rolling out a bit of abstraction to be able to clean
up the master logic in the next step.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_lock.c b/drivers/gpu/drm/drm_lock.c
index ae0a4d3..48ac0eb 100644
--- a/drivers/gpu/drm/drm_lock.c
+++ b/drivers/gpu/drm/drm_lock.c
@@ -219,7 +219,7 @@
 	/* don't set the block all signals on the master process for now 
 	 * really probably not the correct answer but lets us debug xkb
  	 * xserver for now */
-	if (!file_priv->is_master) {
+	if (!drm_is_current_master(file_priv)) {
 		dev->sigdata.context = lock->context;
 		dev->sigdata.lock = master->lock.hw_lock;
 	}