drm/nouveau/kms/nv50-: Explicitly check DPCD backlights for aux enable/brightness

Since we don't support hybrid AUX/PWM backlights in nouveau right now,
let's add some explicit checks so that we don't break nouveau once we
enable support for these backlights in other drivers.
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211105183342.130810-3-lyude@redhat.com

drivers/gpu/drm/nouveau/nouveau_backlight.c

@@ -308,7 +308,10 @@ nv50_backlight_init(struct nouveau_backlight *bl,
 		if (ret < 0)
 			return ret;
 
-		if (drm_edp_backlight_supported(edp_dpcd)) {
+		/* TODO: Add support for hybrid PWM/DPCD panels */
+		if (drm_edp_backlight_supported(edp_dpcd) &&
+		    (edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP) &&
+		    (edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP)) {
 			NV_DEBUG(drm, "DPCD backlight controls supported on %s\n",
 				 nv_conn->base.name);