drm/msm/dp: add linux/io.h header to fix build errors

Add <linux/io.h> header to pull in readl/writel and friends.
This eliminates the following build errors:

drivers/gpu/drm/msm/dp/dp_panel.c: In function 'msm_dp_read_link':
drivers/gpu/drm/msm/dp/dp_panel.c:33:16: error: implicit declaration of function 'readl_relaxed' [-Wimplicit-function-declaration]
   33 |         return readl_relaxed(panel->link_base + offset);
drivers/gpu/drm/msm/dp/dp_panel.c: In function 'msm_dp_write_link':
drivers/gpu/drm/msm/dp/dp_panel.c:43:9: error: implicit declaration of function 'writel' [-Wimplicit-function-declaration]
   43 |         writel(data, panel->link_base + offset);

Fixes: 603fc0fc ("drm/msm/dp: drop the msm_dp_catalog module")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Stephen Boyd <swboyd@chromium.org>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Cc: Dmitry Baryshkov <lumag@kernel.org>
Cc: Rob Clark <robin.clark@oss.qualcomm.com>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/659442/
Link: https://lore.kernel.org/r/20250617185611.2965223-1-rdunlap@infradead.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>