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/drivers/gpu/drm/drm_crtc_helper.c b/drivers/gpu/drm/drm_crtc_helper.c
index 9f8b894..df6a12d 100644
--- a/drivers/gpu/drm/drm_crtc_helper.c
+++ b/drivers/gpu/drm/drm_crtc_helper.c
@@ -337,10 +337,12 @@
 		}
 
 		encoder_funcs = encoder->helper_private;
-		if (!(ret = encoder_funcs->mode_fixup(encoder, mode,
-						      adjusted_mode))) {
-			DRM_DEBUG_KMS("Encoder fixup failed\n");
-			goto done;
+		if (encoder_funcs->mode_fixup) {
+			if (!(ret = encoder_funcs->mode_fixup(encoder, mode,
+							      adjusted_mode))) {
+				DRM_DEBUG_KMS("Encoder fixup failed\n");
+				goto done;
+			}
 		}
 	}