backend/drm: only import buffers once

[emersion]

Right now each time the DRM backend wants to scan-out a client buffer it imports it via GBM. On some drivers I believe this degrades performance. For instance with #2240 and AMD, I end up with the compositor using near 100% CPU, spending a lot of time in Addr::V2::CoordEq::solve in radeonsi.so.

Instead we should import each buffer once, and then re-use the gbm_bo.

[emersion]

The AMD-specific issue may be fixed by https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5398

Would still be a good idea to import buffers once.