presentation-time: use enum entry description tags

Instead of describing each enum entry in the enum description,
use enum entry descriptions. This avoids the awkward list of
flags in the top-level description.

This has been possible for a long time, but wasn't correctly
handled by wayland-scanner until recently [1].

[1]: wayland/wayland!151



Signed-off-by: Simon Ser <contact@emersion.fr>