drm/amd/display: Make function wait_for_alt_mode static

Fix sparse warnings:

drivers/gpu/drm/amd/display/dc/core/dc_link.c:687:6: warning: symbol 'wait_for_alt_mode' was not declared. Should it be static?

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>