drm/amd/display: enable hostvm based on roimmu active for dcn2.1

Enabling hostvm when ROIMMU is not active seems to break GPUVM.
This fixes the issue by not enabling hostvm if ROIMMU is not
activated.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>