disp: msm: add display frequency change notification

Added notifier to inform registered consumers about display changing
frequency.

Change-Id: Ic619b233a03f002862d059bfea671d7b2c5807af
Signed-off-by: daiss1 <daiss1@lenovo.com>
Reviewed-on: https://gerrit.mot.com/1465621
SLTApproved: Slta Waiver
SME-Granted: SME Approvals Granted
Tested-by: Jira Key
Reviewed-by: Guobin Zhang <zhanggb@motorola.com>
Submit-Approved: Jira Key
(cherry picked from commit c69b88c83b5780c3ce858dbf362e0aa4a92b8c67)
Reviewed-on: https://gerrit.mot.com/1467534
Submit-Approved: Qiang Nie <nieqiang@motorola.com>
Tested-by: Qiang Nie <nieqiang@motorola.com>
Reviewed-on: https://gerrit.mot.com/1509822
Reviewed-by: Hua Tan <tanhua1@motorola.com>

msm/dsi/dsi_panel.c

@@ -18,6 +18,10 @@
 #include "sde_dbg.h"
 #include "dsi_display.h"
 
+static struct blocking_notifier_head dsi_freq_head =
+			BLOCKING_NOTIFIER_INIT(dsi_freq_head);
+EXPORT_SYMBOL_GPL(dsi_freq_head);
+
 /**
  * topology is currently defined by a set of following 3 values:
  * 1. num of layer mixers
@@ -4785,6 +4789,10 @@ int dsi_panel_pre_mode_switch_to_video(struct dsi_panel *panel)
 		       panel->name, rc);
 
 	mutex_unlock(&panel->panel_lock);
+	/* notify consumers only if refresh rate has been updated */
+	if (!rc)
+		blocking_notifier_call_chain(&dsi_freq_head,
+			(unsigned long)panel->cur_mode->timing.refresh_rate, NULL);
 	return rc;
 }