drm/amd/display: fix MALL size hardcoded for DCN321

[Why]
MALL size available can vary for different SKUs
MALL size was still hardcoded for DCN321

[How]
Remove hardcoding MALL size for DCN321

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>