drm/amd/amdgpu: store fragment_size in vm_manager

adds fragment_size in the vm_manager structure and
implements hardware setup for it.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>