drm/bridge: analogix_dp: make analogix_dp_start_aux_transaction() static

This eliminates the following sparse warning:

drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c:527:5: warning: symbol
'analogix_dp_start_aux_transaction' was not declared. Should it be
static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200912033843.143240-1-yanaijie@huawei.com