drm/msm: Use drm_attach_bridge() to attach a bridge to an encoder

This is part of our attempt to make the bridge chain a double-linked
list based on the generic list helpers. In order to do that, we must
patch all drivers manipulating the encoder->bridge field directly.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20190826152649.13820-9-boris.brezillon@collabora.com