drm/dp: Enable backlight control using luminance

Add flag to enable brightness control via luminance value
when enabling edp backlight.

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://lore.kernel.org/r/20250620063445.3603086-11-suraj.kandpal@intel.com

Author: surajk8

drivers/gpu/drm/display/drm_dp_helper.c

@@ -4038,6 +4038,9 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
 	else
 		dpcd_buf = DP_EDP_BACKLIGHT_CONTROL_MODE_PWM;
 
+	if (bl->luminance_set)
+		dpcd_buf |= DP_EDP_PANEL_LUMINANCE_CONTROL_ENABLE;
+
 	if (bl->pwmgen_bit_count) {
 		ret = drm_dp_dpcd_write_byte(aux, DP_EDP_PWMGEN_BIT_COUNT, bl->pwmgen_bit_count);
 		if (ret < 0)