drm: fixes crct set_mode when encoder mode_fixup is null.

Avoids null crash when encoders don't implement mode_fixup.

Signed-off-by: Carlos Palminha <palminha@synopsys.com>
[danvet: Also update kerneldoc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455106522-32307-1-git-send-email-palminha@synopsys.com
diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
index a126a0d..b61c2d4 100644
--- a/include/drm/drm_modeset_helper_vtables.h
+++ b/include/drm/drm_modeset_helper_vtables.h
@@ -439,7 +439,7 @@
 	 * can be modified by this callback and does not need to match mode.
 	 *
 	 * This function is used by both legacy CRTC helpers and atomic helpers.
-	 * With atomic helpers it is optional.
+	 * This hook is optional.
 	 *
 	 * NOTE:
 	 *