drm/amdgpu: split the VM entity into direct and delayed

For page fault handling we need to use a direct update which can't be
blocked by ongoing user CS.

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