amdgpu: vamgr can be a struct instead of a pointer

vamgr is an integral part of amdgpu_device. We don't need to calloc and free it.
This can save CPU time, reduce heap fragmentation.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
[Grazvydas Ignotas: rebase, correct a typo in commit message]
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>