drm: Undo damage to page_flip_ioctl

drm: Undo damage to page_flip_ioctl

I screwed up rebasing of my patch in

commit 77f9b424681db479b91f4f545740aa9b8edbcfe1
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Sep 21 10:59:24 2016 +0200

    drm: Extract drm_plane.[hc]

which meant on error paths drm_crtc_vblank_put could be called without
a get, leading to an underrun of the refcount.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98020
Reported-and-tested-by: Andy Furniss <adf.lists@gmail.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161003082827.11586-1-daniel.vetter@ffwll.ch
Signed-off-by: Dave Airlie <airlied@redhat.com>