libweston: Move color manager fallback into weston_compositor_backends_loaded()

Initialize no-op color manager in weston_compositor_backends_loaded()
if weston_compositor_load_color_manager() was never called.

This makes weston_compositor_load_backend() live up to its name and
prepares it to be called multiple times.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>