GPU-DRM: Delete an unnecessary check before drm_property_unreference_blob()

The drm_property_unreference_blob() function tests whether its argument
is NULL and then returns immediately.
Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Zhao Junwang <zhjwpku@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_crtc.c

@@ -4469,9 +4469,7 @@ static int drm_property_replace_global_blob(struct drm_device *dev,
 			goto err_created;
 	}
 
-	if (old_blob)
-		drm_property_unreference_blob(old_blob);
-
+	drm_property_unreference_blob(old_blob);
 	*replace = new_blob;
 
 	return 0;