drm/amd/display: correct hostvm flag

[Why]
Hostvm should be enabled/disabled accordding to
the status of riommu_active, but hostvm always
be disabled on DCN31 which causes underflow

[How]
Set correct hostvm flag on DCN31

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Sherry Wang <Yao.Wang1@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>