drm/amdgpu: meld together VM fragment and huge page handling

This optimizes the generating of PTEs by walking the hierarchy only once
for a range and making changes as necessary.

It allows for both huge (2MB) as well giant (1GB) pages to be used on
Vega and Raven.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>