compositor-drm: drm_set_backlight heads

Fix this function to support more than one head per output.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>

libweston/compositor-drm.c

@@ -3806,13 +3806,15 @@ drm_get_backlight(struct drm_head *head)
 static void
 drm_set_backlight(struct weston_output *output_base, uint32_t value)
 {
-	struct drm_head *head = to_drm_head(weston_output_get_first_head(output_base));
+	struct drm_output *output = to_drm_output(output_base);
+	struct drm_head *head;
 	long max_brightness, new_brightness;
 
-	if (!head->backlight)
+	if (value > 255)
 		return;
 
-	if (value > 255)
+	wl_list_for_each(head, &output->base.head_list, base.output_link) {
+		if (!head->backlight)
 			return;
 
 		max_brightness = backlight_get_max_brightness(head->backlight);
@@ -3821,6 +3823,7 @@ drm_set_backlight(struct weston_output *output_base, uint32_t value)
 		new_brightness = (value * max_brightness) / 255;
 
 		backlight_set_brightness(head->backlight, new_brightness);
+	}
 }
 
 /**