backends/drm: support dmabuf-feedback

dmabuf-feedback allows the compositor to give the clients better feedback on what
formats and modifiers they should use, and for which device they should allocate.
This way they can reallocate for scanout whenever the compositor tells them to,
which makes direct scanout work for a lot more devices and applications.
2 jobs for !1706 with work/dmabuf-feedback in 20 minutes and 38 seconds (queued for 8 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed freebsd_qt515 #165961
FreeBSD

00:08:43

passed suse_tumbleweed_qt515 #165960
Linux

00:20:38