Otherwise we incur an unsightly WARNING. The mutex locking is a bit
overkill, but it curbs races and eventially we might grow a locking
check in the vblank wait code to make sure the right crtc lock is
held.

This is fallout from

commit 93937071
Author:     Jesse Barnes <jbarnes@virtuousgeek.org>
AuthorDate: Fri Apr 4 16:12:09 2014 -0700

    drm/i915: warn when a vblank wait times out

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79612Tested-by: Guo Jinxian <jinxianx.guo@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Rebase on top of drm core ww locking changes.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>