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

drivers/gpu/drm/drm_crtc_helper.c

@@ -337,12 +337,14 @@ bool drm_crtc_helper_set_mode(struct drm_crtc *crtc,
 		}
 
 		encoder_funcs = encoder->helper_private;
+		if (encoder_funcs->mode_fixup) {
 			if (!(ret = encoder_funcs->mode_fixup(encoder, mode,
 							      adjusted_mode))) {
 				DRM_DEBUG_KMS("Encoder fixup failed\n");
 				goto done;
 			}
 		}
+	}
 
 	if (!(ret = crtc_funcs->mode_fixup(crtc, mode, adjusted_mode))) {
 		DRM_DEBUG_KMS("CRTC fixup failed\n");

include/drm/drm_modeset_helper_vtables.h

@@ -439,7 +439,7 @@ struct drm_encoder_helper_funcs {
 	 * 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:
 	 *