drm/sti: Drop of_gpio header

This driver includes the deprecated OF GPIO header <linux/of_gpio.h>
yet fail to use symbols from it, so drop this include.

Cc: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Acked-by: Alain Volmat <avolmat@me.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220812205746.609107-5-mairacanal@riseup.net