backends/drm: improve dmabuf feedback

Commit 1: limit modifiers to those supported by egl. While the format+modifiers supported by drm are usually also supported by egl, there's not really a guarantee for that - and if a client were to use one of those format+modifier pairs, buffer import could fail. That's not necessarily a big deal but it's more efficient to prevent it from happening

Commit 2: Instead of only allowing the current format, send the default tranches modified to only contain formats and modifiers suitable for scanout. In order to not fail when we can't do direct scanout with a given format (because that may require a modeset, which we don't allow), keep a blacklist of attempted formats and modifiers for the current client.