compositor/main: Re-work plug-in loading to avoid an invalid color manager

Rather than loading the plug-ins when loading the DRM backend, do that
after *all* the other backends have been loaded, and after we made sure
we have at least a no-op color manager.

As the plug-ins enable the outputs on their own this has the side-effect
of enabling the output without having any color manager set-up at that
time.  Moving the plug-in loading a bit later ensures that we have one
set-up (a no-op one).

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>