drm: Only handle _DRM_VBLANK_NEXTONMISS once

Consolidate the _DRM_VBLANK_NEXTONMISS handling between drm_wait_vblank
and drm_queue_vblank_event.

This is a cleanup spotted while working on other changes.

(The way it was previously handled could also theoretically result in
drm_queue_vblank_event unnecessarily bumping vblwait->request.sequence,
if the vblank counter happened to increment between the
drm_vblank_count(_and_time) calls in each function, but that's unlikely)

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466755187-29418-1-git-send-email-michel@daenzer.net