drm/i810: Prevent underflow in ioctl

commit 4f69851f upstream.

The "used" variables here come from the user in the ioctl and it can be
negative.  It could result in an out of bounds write.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004102251.GC823@mwanda


Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>