drm/bridge: ti-sn65dsi86: Split connector creation to a function

To prepare for making connector creation option, move connector creation
out of ti_sn_bridge_attach to a separate function.

No functional change intended.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210624000304.16281-7-laurent.pinchart+renesas@ideasonboard.com