drm/amdgpu: add helper for VM PD/PT allocation parameters v3

Add a helper function to figure them out only once.

v2: fix typo with memset
v3: rebase on kfd changes (Alex)

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