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: NHarry Wentland <harry.wentland@amd.com>
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: Nzhengbin <zhengbin13@huawei.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link.c

@@ -685,7 +685,7 @@ static bool is_same_edid(struct dc_edid *old_edid, struct dc_edid *new_edid)
 	return (memcmp(old_edid->raw_edid, new_edid->raw_edid, new_edid->length) == 0);
 }
 
-bool wait_for_alt_mode(struct dc_link *link)
+static bool wait_for_alt_mode(struct dc_link *link)
 {
 
 	/**