[WHY]
Calculations differ with HostVM enabled/disabled, causing underflow in
configs with high refresh displays + scaling due to lower available BW

[HOW]
Check riommu_active in order to pass correct HostVM enablement to DML
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>