drm/hisilicon/hibmc: Include <linux/io.h> for readl() and writel()

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 <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
Signed-off-by: Thomas Zimmermann <[email protected]>
Fixes: 45b64fd9f7ae ("drm/fb-helper: Remove unnecessary include statements")
Cc: Thomas Zimmermann <[email protected]>
Cc: Javier Martinez Canillas <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/dri-devel/[email protected]/T/#u # [1]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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