drm/amdgpu: add amdgpu_vm_entries_mask v2

We can't get the mask for the root directory from the number of entries.

So add a new function to avoid that problem.

v2: fix typo in mask

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>