drm/rockchip: cdn-dp-core: Make cdn_dp_core_suspend/resume static

This fixes the following warning detected when running make with W=1

    drivers/gpu/drm/rockchip//cdn-dp-core.c:1112:5: warning: no previous
    prototype for ‘cdn_dp_suspend’ [-Wmissing-prototypes]

    drivers/gpu/drm/rockchip//cdn-dp-core.c:1126:5: warning: no previous
    prototype for ‘cdn_dp_resume’ [-Wmissing-prototypes]

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200426161653.7710-1-enric.balletbo@collabora.com