drm: rename dev->count_lock to dev->buf_lock

Since really that's all it protects - legacy horror stories in
drm_bufs.c. Since I don't want to waste any more time on this I didn't
bother to actually look at what it protects in there, but it's at
least contained now.

v2: Move the spurious hunk to the right patch (Thierry).

Cc: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>