drm/bridge: display-connector: add DP support

Add DP support to display-connector driver. The driver will support HPD
via a GPIO and DP PWR.

DP PWR will be enabled at probe, which is not optimal, but I'm not sure
what would be a good place to enable and disable DP PWR. Perhaps
attach/detach, but I don't know if enabling HW is something that attach
is supposed to do.

In any case, I don't think there's much difference in power consumption
between the version in this patch and enabling the regulator later: if
the driver probes, supposedly it will attach very soon afterwards, and
we need to enable the DP PWR as soon as possible.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130112919.241054-3-tomi.valkeinen@ti.com