drm/atomic-helper: Make crtc helper funcs optional

Allow drivers to call drm_atomic_helper_check_modeset() without
having the crtc helper funcs specified. i915 doesn't need those
anymore.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708125325.16576-3-ville.syrjala@linux.intel.comReviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>

drivers/gpu/drm/drm_atomic_helper.c

@@ -471,7 +471,7 @@ mode_fixup(struct drm_atomic_state *state)
 			continue;
 
 		funcs = crtc->helper_private;
-		if (!funcs->mode_fixup)
+		if (!funcs || !funcs->mode_fixup)
 			continue;
 
 		ret = funcs->mode_fixup(crtc, &new_crtc_state->mode,