backend-drm: Don't assume DRM_CAP_ATOMIC_ASYNC_PAGE_FLIP is available

If it is, try to use it, otherwise add a new Weston environmental variable to still allow users to enable it on their own.

Fixes: 9203d98f

Signed-off-by: Marius Vlad marius.vlad@collabora.com Reported-by: Simon Ser contact@emersion.fr