drm/i915/backlight: Check Luminance based brightness control for VESA

Check if we are capable of controlling brightness via luminance
which is dependent on PANEL_LUMINANCE_CONTROL_CAPABLE bit being set
on EDP_GENERAL_CAPABILITY_2 register.

--v2
-Prefer using luminance rather than nits [Jani]
-Fix commit message

--v3
-Fix the bit name used in commit message [Arun]
-Use correct edp_dpcd[] to check the capability [Arun]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Tested-by: Ben Kao <ben.kao@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250206063253.2827017-5-suraj.kandpal@intel.com

Author: surajk8

drivers/gpu/drm/i915/display/intel_display_types.h

@@ -413,6 +413,7 @@ struct intel_panel {
 		union {
 			struct {
 				struct drm_edp_backlight_info info;
+				bool luminance_control_support;
 			} vesa;
 			struct {
 				bool sdr_uses_aux;

drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c

@@ -575,6 +575,15 @@ intel_dp_aux_supports_vesa_backlight(struct intel_connector *connector)
 {
 	struct intel_display *display = to_intel_display(connector);
 	struct intel_dp *intel_dp = intel_attached_dp(connector);
+	struct intel_panel *panel = &connector->panel;
+
+	if ((intel_dp->edp_dpcd[3] & DP_EDP_PANEL_LUMINANCE_CONTROL_CAPABLE)) {
+		drm_dbg_kms(display->drm,
+			    "[CONNECTOR:%d:%s] AUX Luminance Based Backlight Control Supported!\n",
+			    connector->base.base.id, connector->base.name);
+		panel->backlight.edp.vesa.luminance_control_support = true;
+		return true;
+	}
 
 	if (drm_edp_backlight_supported(intel_dp->edp_dpcd)) {
 		drm_dbg_kms(display->drm,