drm: bridge: nwl-dsi: Drop panel_bridge from nwl_dsi

panel_bridge pointer never used anywhere except the one it
looked up at nwl_dsi_bridge_attach.

Drop it from the nwl_dsi structure.

Reviewed-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220110172533.66614-1-jagan@amarulasolutions.com