backend/drm: use atomic test-only commits for modifier fallback

Instead of trying to perform a real modeset in init_renderer,
perform an atomic test-only commit to find out whether disabling
modifiers is necessary because of bandwidth limitations.

This decouples init_renderer from the actual commit, making it
possible to modeset an output with a user-supplied buffer instead
of a black frame.

We loose the ability to make sure the buffers coming from the
swapchain will work fine when using the legacy interface. This
can break i915 when atomic is disabled and modifiers enabled.
But i915 always has atomic (so the user must explicitly disable it
to run into potential bandwidth limitations) and is the only known
problematic driver.

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