backend-drm: Enable atomic async flip support

Since DRM_CAP_ATOMIC_ASYNC_PAGE_FLIP capability is available in the
mainline kernel, now we should enable back tearing support.

v2:

 - Bump kernel version to 6.9
 - include the fallback definitions

Reviewed-by: Marius Vlad <marius.vlad@collabora.com>
Reviewed-by: Derek Foreman <derek.foreman@collabora.com>
Signed-off-by: Naveen Kumar <naveen1.kumar@intel.com>