drm: bridge: dw-hdmi: disable scamble when tmdsclk less than 340Mhz

to Improve signal compatibility disable scamble when tmdsclk less than
340Mhz by default. and can enable it by define "scramble-low-rates;"
in dts file.

Change-Id: I0bd5d8e2ea4df065d84018615d4c39cac7ac441a
Signed-off-by: xuhuicong <xhc@rock-chips.com>

Documentation/devicetree/bindings/display/rockchip/dw_hdmi-rockchip.txt

@@ -44,6 +44,7 @@ Optional properties
 - unsupported-yuv-input: If yuv input is not supported, set this property.
 - unsupported-deep-color: If deep color mode is not supported, set this property.
 - hdcp1x-enable: enable hdcp1.x, enable if defined, disable if not defined
+- scramble-low-rates: if defined enable scarmble when tmdsclk less than 340Mhz
 
 Example:
 

drivers/gpu/drm/bridge/synopsys/dw-hdmi.c

@@ -267,6 +267,7 @@ struct dw_hdmi {
 	unsigned int audio_cts;
 	unsigned int audio_n;
 	bool audio_enable;
+	bool scramble_low_rates;
 
 	struct extcon_dev *extcon;
 
@@ -2150,7 +2151,8 @@ static void hdmi_av_composer(struct dw_hdmi *hdmi,
 	/* Scrambling Control */
 	if (dw_hdmi_support_scdc(hdmi)) {
 		if (vmode->mtmdsclock > HDMI14_MAX_TMDSCLK ||
-		    hdmi_info->scdc.scrambling.low_rates) {
+		    (hdmi_info->scdc.scrambling.low_rates &&
+		     hdmi->scramble_low_rates)) {
 			/*
 			 * HDMI2.0 Specifies the following procedure:
 			 * After the Source Device has determined that
@@ -3868,6 +3870,9 @@ __dw_hdmi_probe(struct platform_device *pdev,
 
 	dw_hdmi_register_debugfs(dev, hdmi);
 
+	if (of_property_read_bool(np, "scramble-low-rates"))
+		hdmi->scramble_low_rates = true;
+
 	if (of_property_read_bool(np, "hdcp1x-enable"))
 		hdcp1x_enable = 1;
 	dw_hdmi_register_hdcp(dev, hdmi, val, hdcp1x_enable);