drm: rip out dev->ioctl_count tracking

Now dev->ioctl_count tries to prevent the device from disappearing if
it's still in use. And if we'd actually need this code it would be
hopelessly racy and broken.

But luckily the vfs already takes care of this. So we can just rip it
out.

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