drm/amdgpu: allow specifying vm_block_size for multi level PDs v2

This patch allows specifying the vm_block_size even when multi level
page directories are active.

v2: fix signed/unsigned compare warning

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