drm/rockchip: dw_hdmi: fix incorrect clock in vpll clock error message

Error message incorrectly refers to grf clock instead of vpll clock.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201024035321.4898-1-net147@gmail.com