sunxi: g2d: don't skip mutex_unlock on invalid ioctl

Signed-off-by: Jens Kuske <jenskuske@gmail.com>
sunxi: disp: Allow negative hardware cursor position

The hardware cursor coordinates seem to be signed 14 bit values.
This has been tested on sun4i and sun7i.

Signed-off-by: Jens Kuske <jenskuske@gmail.com>