drm/amd/display: Set hvm_enabled flag for S/G mode

[Why]
After enabling S/G on dcn314 a screen corruption may be observed.
HostVM flag should be set in S/G mode to be included in DML calculations.

[How]
In S/G mode gpu_vm_support flag is set.
Use its value to init is_hvm_enabled.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>