backend-drm: Add support for handling release_fence

If DRM_CAP_RELEASE_FENCE capability is exposed by KMS, then include the RELEASE_FENCE_PTR as part of the atomic commit. After getting a valid release fence fd, take an additional reference on all the fbs included in the atomic commit and also register a handler for the release fence fd by adding it to the main event loop.

This patch in addition to other related DRM driver patches can solve: #514 (closed)

Corresponding DRM/Virtio-gpu patches: https://lists.freedesktop.org/archives/dri-devel/2021-September/323158.html

Qemu UI patch: https://lists.nongnu.org/archive/html/qemu-devel/2021-09/msg03464.html