Include <linux/io.h> to get readl() and writel() on S390. The error
message is shown below and a bug report is at [1].

  drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_vdac.c:75:15: error: implicit declaration of function 'readl' [-Werror=implicit-function-declaration]
     75 |         reg = readl(priv->mmio + HIBMC_DISPLAY_CONTROL_HISILE);
        |               ^~~~~
  drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_vdac.c:80:9: error: implicit declaration of function 'writel' [-Werror=implicit-function-declaration]
     80 |         writel(reg, priv->mmio + HIBMC_DISPLAY_CONTROL_HISILE);
	|         ^~~~~~
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 45b64fd9 ("drm/fb-helper: Remove unnecessary include statements")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Link: https://lore.kernel.org/dri-devel/202211060608.qrTg8b2E-lkp@intel.com/T/#u # [1]
Link: https://patchwork.freedesktop.org/patch/msgid/20221107125329.12842-3-tzimmermann@suse.de