drm/bridge: synopsys: dw-mipi-dsi: Add dw_mipi_dsi_get_bridge() helper

Add dw_mipi_dsi_get_bridge() helper so that it can be used by vendor
drivers which implement vendor specific extensions to Synopsys DW MIPI DSI.

Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230821034008.3876938-2-victor.liu@nxp.com