drm/amdgpu: fix a possible null pointer dereference

In amdgpu_connector_add_common_modes(), the return value of drm_cvt_mode()
is assigned to mode, which will lead to a NULL pointer dereference on
failure of drm_cvt_mode(). Add a check to avoid npd.

Cc: stable@vger.kernel.org
Fixes: d38ceaf9 ("drm/amdgpu: add core driver (v4)")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c

@@ -442,6 +442,9 @@ static void amdgpu_connector_add_common_modes(struct drm_encoder *encoder,
 			continue;
 
 		mode = drm_cvt_mode(dev, common_modes[i].w, common_modes[i].h, 60, false, false, false);
+		if (!mode)
+			return;
+
 		drm_mode_probed_add(connector, mode);
 	}
 }