amdgpu: fix build on non-DCN platforms.

This fixes the build here locally on my 32-bit arm build.

Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -9497,15 +9497,15 @@ static int amdgpu_dm_atomic_check(struct drm_device *dev,
 	struct drm_connector_state *old_con_state, *new_con_state;
 	struct drm_crtc *crtc;
 	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
-	struct drm_dp_mst_topology_mgr *mgr;
-	struct drm_dp_mst_topology_state *mst_state;
 	struct drm_plane *plane;
 	struct drm_plane_state *old_plane_state, *new_plane_state;
 	enum dc_status status;
 	int ret, i;
 	bool lock_and_validation_needed = false;
 	struct dm_crtc_state *dm_old_crtc_state, *dm_new_crtc_state;
 #if defined(CONFIG_DRM_AMD_DC_DCN)
+	struct drm_dp_mst_topology_mgr *mgr;
+	struct drm_dp_mst_topology_state *mst_state;
 	struct dsc_mst_fairness_vars vars[MAX_PIPES];
 #endif