drm/bridge: Refactor out the panel wrapper from the lvds-encoder bridge.

Many DRM drivers have common code to make a stub connector
implementation that wraps a drm_panel.  By wrapping the panel in a DRM
bridge, all of the connector code (including calls during encoder
enable/disable) goes away.

v2: Fix build with CONFIG_DRM=m, drop "dev" argument that should just
    be the panel's dev, move kerneldoc up a level and document
    _remove().
v3: Fix another breakage with CONFIG_DRM=m, fix breakage with
    CONFIG_OF=n, move protos under CONFIG_DRM_PANEL_BRIDGE, wrap a
    line.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com> (v2)
Acked-by: Archit Taneja <architt@codeaurora.org> (v2)
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170602202514.11900-1-eric@anholt.net