drm: Kill file_priv->ioctl_count tracking

It's racy, and it's only used in debugfs. There are simpler ways to
know whether something is going on (like looking at dmesg with full
debugging enabled). And they're all much more useful.

So let's just rip this out.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>