drm/rockchip: dw_hdmi: allow including external phys

Some variants of the dw-hdmi on Rockchip socs use a separate phy block
accessed via the generic phy framework, so allow them to be included
if such a phy reference is found.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Zheng Yang <zhengyang@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180912124740.20343-5-heiko@sntech.de