backends/drm: add support for drm plane size hints

On a lot of hardware, using bigger plane sizes than necessary means wasting power. This is specifically problematic with the cursor plane, where we so far only had a single fixed size hint through drm caps, even though the hardware often could use a smaller cursor size. This adds support for the per-plane SIZE_HINTS property, which allows us to pick a smaller cursor size when the cursor image fits into it, and should save some power that way.