drm/amdgpu: allocate VM PDs/PTs on demand

Let's start to allocate VM PDs/PTs on demand instead of pre-allocating
them during mapping.

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