drm: omapdrm: Delete surplus GPIO includes

The OMAP DRM driver includes <linux/gpio.h> into the two
hdmi4.c and hdmi5.c files but does not use any symbols from
these files. Drop the includes.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200626220606.340937-1-linus.walleij@linaro.org