drm/bridge/tfp410: Make symbol tfp410_platform_driver static

Fixes the following sparse warning:

drivers/gpu/drm/bridge/ti-tfp410.c:223:24: warning:
 symbol 'tfp410_platform_driver' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170209152549.30711-1-weiyj.lk@gmail.com