drm/i915: Use the correct max source link rate for MST

[why]
Previously used value was not safe to provide the correct value, i.e. it
could be 0 if not not configured, leading to no MST on this platform.

[how]
Do not use the value from BIOS, but from the structure populated at
encoder initialization time.

Fixes: 98025a62 ("drm/dp_mst: Use Extended Base Receiver Capability DPCD space")
Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[fixed open coded drm_dp_link_rate_to_bw_code()]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430214531.24565-2-nikola.cornij@amd.com

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

@@ -942,7 +942,8 @@ intel_dp_mst_encoder_init(struct intel_digital_port *dig_port, int conn_base_id)
 	struct intel_dp *intel_dp = &dig_port->dp;
 	enum port port = dig_port->base.port;
 	int ret;
-	int bios_max_link_rate;
+	int max_source_rate =
+		intel_dp->source_rates[intel_dp->num_source_rates - 1];
 
 	if (!HAS_DP_MST(i915) || intel_dp_is_edp(intel_dp))
 		return 0;
@@ -957,11 +958,11 @@ intel_dp_mst_encoder_init(struct intel_digital_port *dig_port, int conn_base_id)
 
 	/* create encoders */
 	intel_dp_create_fake_mst_encoders(dig_port);
-	bios_max_link_rate = intel_bios_dp_max_link_rate(&dig_port->base);
 	ret = drm_dp_mst_topology_mgr_init(&intel_dp->mst_mgr, &i915->drm,
 					   &intel_dp->aux, 16, 3,
 					   (u8)dig_port->max_lanes,
-					   (u8)(bios_max_link_rate / 27000), conn_base_id);
+					   drm_dp_link_rate_to_bw_code(max_source_rate),
+					   conn_base_id);
 	if (ret)
 		return ret;