drm/amd/display: DC VM Fixes

[Why]
VM_helper needs to be intialized with the dc struct in order to fix
an unallocated memory issue. System aperture settings should be
initialized to 0 and guarded with a check to make sure vm_config
is valid.

[How]
Allocate and free memory for vm_helper with other dc members.
Check whether the vm_config valid bit is set before initializing
aperture settings.

Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>