xf86-video-modesetting: Add ms_queue_vblank helper [v3]

This provides an API wrapper around the kernel interface for queueing
a vblank event, simplifying all of the callers.

v2: Fix missing '|' in computing vbl.request.type

v3: Remove spurious bit of next patch (thanks, Michel Dänzer)

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 677c32bc)