drm/bridge/synopsys: dsi: make dw_mipi_dsi_bridge_mode_set() static

sparse complains:

drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c:703:6: warning: symbol
'dw_mipi_dsi_bridge_mode_set' was not declared. Should it be static?

Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180109203319.139520-1-briannorris@chromium.org