drm/bridge: dw-hdmi: allow forcing vendor phy-type

In some IP implementations the reading of the phy-type may be broken.
One example are the Rockchip rk3228 and rk3328 socs that use a separate
vendor-type phy from Innosilicon but still report the HDMI20_TX type.

So allow the glue driver to force the vendor-phy for these cases.
In the future it may be necessary to allow forcing other types, but
for now we'll keep it simply to the case actually seen in the wild.

changes in v3:
- only allow forcing vendor type, as suggested by Laurent

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Zheng Yang <zhengyang@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180912124740.20343-2-heiko@sntech.de
Link: https://patchwork.freedesktop.org/patch/msgid/20180912124740.20343-3-heiko@sntech.de