drm/rockchip: dw_hdmi: Allow outputs that don't need output switching

So far we always encountered socs with 2 output crtcs needing the driver
to tell the hdmi block which output to connect to. But there also exist
socs with only one crtc like the rk3228, rk3328 and rk3368.

So adapt the register field to simply carry a negative value to signal
that no output-switching is necessary.

changes in v3:
- fixed wording issue found by Robin Murphy

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/20180912124740.20343-3-heiko@sntech.de