Device-wide modesets with fallbacks

[emersion]

When modesetting an output, perform device-wide atomic test-only commits:

• Try allocating a buffer with modifiers
• If that doesn't work, try allocating a buffer without modifiers
• If that doesn't work, try re-allocating buffers for the other CRTCs without modfiiers
• If that doesn't work, try falling back to lower modes

It's not yet clear how much of this logic should be in the backend and how much should be left to the compositor.