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>