drm/i915: Readout conn_state->max_bpc

Populate conn_state->max_bpc with something sensible from the start.
Otherwise it's possible that we get to compute_sink_pipe_bpp() with
max_bpc==0.

The specific scenario goes as follows:
1. Initial connector state allocated with max_bpc==0
2. Trigger a modeset on the crtc feeding the connector, without
   actually adding the connector to the commit
3. drm_atomic_connector_check() is skipped because the
   connector has not yet been added, hence conn_state->max_bpc
   retains its current value
4. drm_atomic_helper_check_modeset() ->
   drm_atomic_add_affected_connectors() -> the connector
   is now part of the commit
5. compute_baseline_pipe_bpp() -> MISSING_CASE(max_bpc==0)

Note that pipe_bpp itself may not be populated on pre-g4x machines,
in which case we just fall back to max_bpc==8 and let .compute_config()
limit the resulting pipe_bpp further if necessary.

Cc: Daniel Vetter <daniel@ffwll.ch>
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210216160035.4780-1-ville.syrjala@linux.intel.comTested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>

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

@@ -7991,19 +7991,27 @@ static void intel_modeset_update_connector_atomic_state(struct drm_device *dev)
 
 	drm_connector_list_iter_begin(dev, &conn_iter);
 	for_each_intel_connector_iter(connector, &conn_iter) {
-		if (connector->base.state->crtc)
+		struct drm_connector_state *conn_state = connector->base.state;
+		struct intel_encoder *encoder =
+			to_intel_encoder(connector->base.encoder);
+
+		if (conn_state->crtc)
 			drm_connector_put(&connector->base);
 
-		if (connector->base.encoder) {
-			connector->base.state->best_encoder =
-				connector->base.encoder;
-			connector->base.state->crtc =
-				connector->base.encoder->crtc;
+		if (encoder) {
+			struct intel_crtc *crtc =
+				to_intel_crtc(encoder->base.crtc);
+			const struct intel_crtc_state *crtc_state =
+				to_intel_crtc_state(crtc->base.state);
+
+			conn_state->best_encoder = &encoder->base;
+			conn_state->crtc = &crtc->base;
+			conn_state->max_bpc = (crtc_state->pipe_bpp ?: 24) / 3;
 
 			drm_connector_get(&connector->base);
 		} else {
-			connector->base.state->best_encoder = NULL;
-			connector->base.state->crtc = NULL;
+			conn_state->best_encoder = NULL;
+			conn_state->crtc = NULL;
 		}
 	}
 	drm_connector_list_iter_end(&conn_iter);