drm/amdgpu: allow get_vm_pde to change flags as well

And also provide the level for which we need a PDE.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>