drm/atomic: fix null pointer access to mode_fixup callback

This patch fixes null pointer access incurred when
encoder driver didn't set its own mode_fixup callback.

mode_fixup callback shoudn't be called if the callback
of drm_encoder_helper_funcs is NULL.

Changelog v2:
- change it to else if
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_atomic_helper.c

@@ -299,7 +299,7 @@ mode_fixup(struct drm_atomic_state *state)
 						 encoder->base.id, encoder->name);
 				return ret;
 			}
-		} else {
+		} else if (funcs->mode_fixup) {
 			ret = funcs->mode_fixup(encoder, &crtc_state->mode,
 						&crtc_state->adjusted_mode);
 			if (!ret) {