drm: drop redundant drm_file->is_master

The drm_file->is_master field is redundant as it's equivalent to:
    drm_file->master && drm_file->master == drm_file->minor->master

1) "=>"
  Whenever we set drm_file->is_master, we also set:
      drm_file->minor->master = drm_file->master;

  Whenever we clear drm_file->is_master, we also call:
      drm_master_put(&drm_file->minor->master);
  which implicitly clears it to NULL.

2) "<="
  minor->master cannot be set if it is non-NULL. Therefore, it stays as
  is unless a file drops it.

  If minor->master is NULL, it is only set by places that also adjust
  drm_file->is_master.

Therefore, we can safely drop is_master and replace it by an inline helper
that matches:
    drm_file->master && drm_file->master == drm_file->minor->master

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
diff --git a/drivers/gpu/drm/drm_stub.c b/drivers/gpu/drm/drm_stub.c
index 233ea20..18c9b3d 100644
--- a/drivers/gpu/drm/drm_stub.c
+++ b/drivers/gpu/drm/drm_stub.c
@@ -177,7 +177,7 @@
 	int ret = 0;
 
 	mutex_lock(&dev->master_mutex);
-	if (file_priv->is_master)
+	if (drm_is_master(file_priv))
 		goto out_unlock;
 
 	if (file_priv->minor->master) {
@@ -191,13 +191,10 @@
 	}
 
 	file_priv->minor->master = drm_master_get(file_priv->master);
-	file_priv->is_master = 1;
 	if (dev->driver->master_set) {
 		ret = dev->driver->master_set(dev, file_priv, false);
-		if (unlikely(ret != 0)) {
-			file_priv->is_master = 0;
+		if (unlikely(ret != 0))
 			drm_master_put(&file_priv->minor->master);
-		}
 	}
 
 out_unlock:
@@ -211,7 +208,7 @@
 	int ret = -EINVAL;
 
 	mutex_lock(&dev->master_mutex);
-	if (!file_priv->is_master)
+	if (!drm_is_master(file_priv))
 		goto out_unlock;
 
 	if (!file_priv->minor->master)
@@ -221,7 +218,6 @@
 	if (dev->driver->master_drop)
 		dev->driver->master_drop(dev, file_priv, false);
 	drm_master_put(&file_priv->minor->master);
-	file_priv->is_master = 0;
 
 out_unlock:
 	mutex_unlock(&dev->master_mutex);