backends/drm: add stricter checks for direct scanout
Buffers with implicit modifiers from another GPU must not be imported, as the layouts may not be compatible. This effectively means that we have to reject all imports for direct scanout with implicit modifiers whenever we have more than one GPU, because there's no way to detect which GPU a buffer comes from afaict.
For buffers with incompatible modifiers, direct scanout can also be rejected early, saving some computational power.