drm/amd/display: Get backlight controller id from link

[Why]
dc_link_set_backlight_level can be called from a context
where the stream is unknown.  In this case, we can still
find which controller is driving this particular backlight

[How]
Compare links for equality instead of streams
Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -2173,11 +2173,11 @@ bool dc_link_set_backlight_level(const struct dc_link *link,
 			backlight_pwm_u16_16, backlight_pwm_u16_16);
 
 	if (dc_is_embedded_signal(link->connector_signal)) {
-		if (stream != NULL) {
 		for (i = 0; i < MAX_PIPES; i++) {
+			if (core_dc->current_state->res_ctx.pipe_ctx[i].stream) {
 				if (core_dc->current_state->res_ctx.
-						pipe_ctx[i].stream
-						== stream)
+						pipe_ctx[i].stream->sink->link
+						== link)
 					/* DMCU -1 for all controller id values,
 					 * therefore +1 here
 					 */