drm/msm/dsi: Allow dsi to connect to an external bridge

There are platforms where the DSI output can be connected to another
encoder bridge chip (DSI to HDMI, DSI to LVDS etc).

Add support for external bridge support to the dsi driver. We assume that
the external bridge chip would be of the type drm_bridge. The dsi driver's
internal drm_bridge (msm_dsi->bridge) is linked to the external bridge's
drm_bridge struct.

In the case we're connected to an external bridge, we don't need to create
and manage a connector within our driver, it's the bridge driver's
responsibility to create one.

v2:
- Move the external bridge attaching stuff to dsi manager to make things
  cleaner.
- Force the bridge to connect to a video mode encoder for now (the dsi
  mode flags may have not been populated by modeset_init)

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>