backends/drm: set scaling mode to full aspect with generated modes on internal displays
While the scaling mode has caused some issues with external displays, we've had several reports that using "unify outputs" has caused the internal display to no longer show anything, as it changes to an unsupported mode. This sets the scaling mode so that the driver handles the scaling internally, instead of leaving it up to the panel, and it only does so on internal displays with generated modes, to minimize the risk of further breakage.
For 6.2 I hope to get rid of both generated modes and the current way of "mirroring" outputs, but for 6.1 this should at least ensure there's no black screens on laptops anymore.