drm/amd/display: make virtual_disable_link_output static

It's not used outside of virtual_link_hwss.c. Fixes
a -Wmissing-prototypes warning.

Reviewed-by: Harry Wentland <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/virtual/virtual_link_hwss.c

···
 {
 }
 
-void virtual_disable_link_output(struct dc_link *link,
+static void virtual_disable_link_output(struct dc_link *link,
 	const struct link_resource *link_res,
 	enum signal_type signal)
 {