drm/bridge: nwl-dsi: Fix a build warning'nwl_dsi_bridge_mode_fixup' defined but not used

Refer to 44cfc623

 ("drm/bridge: Add NWL MIPI DSI host controller support")
nwl_dsi_bridge_mode_fixup has been set as the callback function of .mode_fixup,
so this callback is added

Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>