driver/display: add set backlight register function

For FT8006S HLT LCD because the FT8006S IC spec backlight register,
we add the mipi_dsi_dcs_set_display_brightness_2bytes_2th_low4bit the
function.

Change-Id: Ib77bf0981ace8c4f9cf77c69a70696ec2075fcae
Reviewed-on: https://gerrit.mot.com/2061265
SME-Granted: SME Approvals Granted
SLTApproved: Slta Waiver
Tested-by: Jira Key
Reviewed-by: Suting Xie <xiest1@motorola.com>
Submit-Approved: Suting Xie <xiest1@motorola.com>

drivers/gpu/drm/drm_mipi_dsi.c

@@ -1133,6 +1133,29 @@ int mipi_dsi_dcs_set_display_brightness_2bytes(struct mipi_dsi_device *dsi,
 }
 EXPORT_SYMBOL(mipi_dsi_dcs_set_display_brightness_2bytes);
 
+/**
+ * mipi_dsi_dcs_set_display_brightness_2bytes_2th_low4bit() - sets the brightness value of
+ *    the display with 2bytes and low 4bit at 2th byte value
+ * @dsi: DSI peripheral device
+ * @brightness: brightness value
+ *
+ * Return: 0 on success or a negative error code on failure.
+ */
+int mipi_dsi_dcs_set_display_brightness_2bytes_2th_low4bit(struct mipi_dsi_device *dsi,
+					u16 brightness)
+{
+	u8 payload[2] = { brightness >> 0x04, brightness & 0x0f };
+	ssize_t err;
+
+	err = mipi_dsi_dcs_write(dsi, MIPI_DCS_SET_DISPLAY_BRIGHTNESS,
+				payload, sizeof(payload));
+	if (err < 0)
+		return err;
+
+	return 0;
+}
+EXPORT_SYMBOL(mipi_dsi_dcs_set_display_brightness_2bytes_2th_low4bit);
+
 /**
  * mipi_dsi_dcs_set_display_brightness_large() - sets the 16-bit brightness value
  *    of the display

include/drm/drm_mipi_dsi.h

@@ -292,6 +292,8 @@ int mipi_dsi_dcs_get_display_brightness(struct mipi_dsi_device *dsi,
 					u16 *brightness);
 int mipi_dsi_dcs_set_display_brightness_2bytes(struct mipi_dsi_device *dsi,
 					u16 brightness);
+int mipi_dsi_dcs_set_display_brightness_2bytes_2th_low4bit(struct mipi_dsi_device *dsi,
+					u16 brightness);
 int mipi_dsi_dcs_set_display_brightness_large(struct mipi_dsi_device *dsi,
 					     u16 brightness);
 int mipi_dsi_dcs_get_display_brightness_large(struct mipi_dsi_device *dsi,