drm/msm/dsi: external bridge support

In order for dsi to support external bridge chips, the dsi driver needs to
check whether the dsi host's child specified in DT is a bridge or a panel.

We first look for a panel node, if we don't have one, we look for a bridge. If
we connect to an external bridge, we don't need to worry about creating a
connector, the bridge driver is responsible for that.

Signed-off-by: Archit Taneja <architt@codeaurora.org>

Conflicts:
	drivers/gpu/drm/msm/dsi/dsi.c
	drivers/gpu/drm/msm/dsi/dsi_manager.c