drm/amd/display: Reduce expected sdp bandwidth for dcn321

[Description]
Modify soc BB to reduce expected sdp bandwidth and align with measurements to
fix underflow issues.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Dillon Varone <Dillon.Varone@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>