drm/amdgpu: add new unlocked flag for PTE updates

For HMM support we need the ability to invalidate PTEs from
a MM callback where we can't lock the root PD.

Add a new flag to better support this instead of assuming
that all invalidation updates are unlocked.

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>