drm/msm/mdp4: Drop unused GPIO include

This file is not using any symbols from <linux/gpio.h> so just
drop this include.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>