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