st/nine: Fixes warning about implicit conversion

Makes the conversion explicit.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=102542



Signed-off-by: Axel Davy <davyaxel0@gmail.com>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>

CC: "17.3 18.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit f61e9a95)