drm/vblank: update recently added vbl interface to be more future proof.

This makes the interface a bit cleaner by leaving a single gap in the
vblank bit space instead of creating two gaps.

Suggestions from Michel on mailing list/irc.

Reviewed-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>