drm/amd/display: Make dc_link_detect_helper static

Fix sparse warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link.c:746:6:
 warning: symbol 'dc_link_detect_helper' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>