drm/amd/display: Update bounding box for DCN3.1

[Why & How]
We're missing a default value for dram_channel_width_bytes in the
DCN3.1 SOC bounding box and we don't currently have the interface in
place to query the actual value from VBIOS.

Put in a hardcoded default until we have the interface in place.
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn31/dcn31_resource.c

@@ -220,6 +220,7 @@ struct _vcs_dpi_soc_bounding_box_st dcn3_1_soc = {
 	.sr_exit_z8_time_us = 402.0,
 	.sr_enter_plus_exit_z8_time_us = 520.0,
 	.writeback_latency_us = 12.0,
+	.dram_channel_width_bytes = 4,
 	.round_trip_ping_latency_dcfclk_cycles = 106,
 	.urgent_latency_pixel_data_only_us = 4.0,
 	.urgent_latency_pixel_mixed_with_vm_data_us = 4.0,