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

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>