drm/amdgpu: fix parameter documentation for amdgpu_vm_free_pts

The function was modified without updating the documentation.

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>