drm/sti: Include the right header

The sti_hdmi.c file include <linux/of_gpio.h> despite not even
using any GPIOs.

What it does use is devm_ioremap_nocache() which comes from
<linux/io.h> implicitly by including that header.

Fix this up by including the right header instead.

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823071428.6155-1-linus.walleij@linaro.org