omapfb/dss: Include the right header

The hdmi4.c and hdmi5.c files include the legacy GPIO
header <linux/gpio.h> but does not use any of the symbols
from this file.

What it does use is the implicit inclusion of <linux/of.h>
leading to compile errors if we just drop this include.

Include the right header.

Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200706125931.752539-1-linus.walleij@linaro.org