drm/nouveau/kms: Cache DP encoders in nouveau_connector

Post-NV50, the only kind of encoder you'll find for DP connectors on Nvidia
GPUs are SORs (serial output resources). Because SORs have fixed
associations with their connectors, we can correctly assume that any DP
connector on a nvidia GPU will have exactly one SOR encoder routed to it
for DisplayPort.

Since we're going to need to be able to retrieve this fixed SOR DP encoder
much more often as a result of hooking up MST helpers for tracking
SST<->MST transitions in atomic states, let's simply cache this encoder in
nouveau_connector for any DP connectors on the system to avoid looking it
up each time. This isn't safe for NV50 since PIORs then come into play,
however there's no code pre-NV50 that would need to look this up anyhow -
so it's not really an issue.
Signed-off-by: Lyude Paul <lyude@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220817193847.557945-12-lyude@redhat.com

drivers/gpu/drm/nouveau/nouveau_connector.c

@@ -1368,7 +1368,7 @@ nouveau_connector_create(struct drm_device *dev,
 			return ERR_PTR(-ENOMEM);
 		}
 		drm_dp_aux_init(&nv_connector->aux);
-		fallthrough;
+		break;
 	default:
 		funcs = &nouveau_connector_funcs;
 		break;
@@ -1431,6 +1431,8 @@ nouveau_connector_create(struct drm_device *dev,
 
 	switch (type) {
 	case DRM_MODE_CONNECTOR_DisplayPort:
+		nv_connector->dp_encoder = find_encoder(&nv_connector->base, DCB_OUTPUT_DP);
+		fallthrough;
 	case DRM_MODE_CONNECTOR_eDP:
 		drm_dp_cec_register_connector(&nv_connector->aux, connector);
 		break;

drivers/gpu/drm/nouveau/nouveau_connector.h

@@ -128,6 +128,9 @@ struct nouveau_connector {
 
 	struct drm_dp_aux aux;
 
+	/* The fixed DP encoder for this connector, if there is one */
+	struct nouveau_encoder *dp_encoder;
+
 	int dithering_mode;
 	int scaling_mode;