drm/rockchip: mark symbols static where possible

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/rockchip/rockchip_drm_drv.c:309:6: warning: no previous prototype for 'rockchip_drm_fb_suspend' [-Wmissing-prototypes]
drivers/gpu/drm/rockchip/rockchip_drm_drv.c:318:6: warning: no previous prototype for 'rockchip_drm_fb_resume' [-Wmissing-prototypes]

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
So this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1474789388-3284-1-git-send-email-baoyou.xie@linaro.org