drm/bridge: ti-sn65dsi86: Avoid possible buffer overflow

Smatch error:buffer overflow 'ti_sn_bridge_refclk_lut' 5 <= 5.

Fixes: cea86c5b ("drm/bridge: ti-sn65dsi86: Implement the pwm_chip")
Signed-off-by: Su Hui <suhui@nfschina.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230608012443.839372-1-suhui@nfschina.com