drm/dp: Sort includes alphabetically

Keeping the list sorted alphabetically makes it much easier to determine
where to add new includes.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021143437.1477719-2-thierry.reding@gmail.com