drm/amd/display: enable vm by default for rn.

[Why & How]
vm should be enabled by default for rn to get
right dml.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>