drm/rockchip: cdn-dp: change rk3399_cdn_dp from global to static

Smatch reports this issue
cdn-dp-core.c:51:20: warning: symbol 'rk3399_cdn_dp' was not declared. Should it be static?

rk3399_cdn_dp is only used in cdn-dp-core.c so change
its storge-class specifier to static.
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220421144304.586396-1-trix@redhat.com

drivers/gpu/drm/rockchip/cdn-dp-core.c

@@ -48,7 +48,7 @@ struct cdn_dp_data {
 	u8 max_phy;
 };
 
-struct cdn_dp_data rk3399_cdn_dp = {
+static struct cdn_dp_data rk3399_cdn_dp = {
 	.max_phy = 2,
 };