backend-drm: add fallback-mode for outputs without modes

If a monitor has broken or invalid EDID, Weston will not be able to populate the modes on the output and fail to enable the output.

To handle this situation, a fixed modeline can be set for such outputs using the mode= option in weston.ini. However, this modeline will always be used even if a monitor with correct EDID is connected. In this case, a different mode might be preferable over the fixed modeline.

Add a fallback-mode= option that accepts a modeline that is only used, if Weston failed to populate the modes from the EDID.

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