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>

drivers/gpu/drm/vmwgfx/vmwgfx_drv.c

@@ -990,7 +990,7 @@ static struct vmw_master *vmw_master_check(struct drm_device *dev,
 	if (unlikely(ret != 0))
 		return ERR_PTR(-ERESTARTSYS);
 
-	if (file_priv->is_master) {
+	if (drm_is_master(file_priv)) {
 		mutex_unlock(&dev->master_mutex);
 		return NULL;
 	}