drm/radeon: fix null pointer dereference in radeon_add_common_modes

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

Cc: stable@vger.kernel.org
Fixes: d50ba256b5 ("drm/kms: start adding command line interface using fb.")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_connectors.c

@@ -505,6 +505,9 @@ static void radeon_add_common_modes(struct drm_encoder *encoder, struct drm_conn
 			continue;
 
 		mode = drm_cvt_mode(dev, common_modes[i].w, common_modes[i].h, 60, false, false, false);
+		if (!mode)
+			continue;
+
 		drm_mode_probed_add(connector, mode);
 	}
 }