drm/amdgpu: Avoid endless loop in GPUVM fragment processing

Don't bounce back to the root level for fragment processing, because
huge pages are not supported at that level. This is unlikely to happen
with the default VM size on Vega, but can be exposed by limiting the
VM size with the amdgpu.vm_size module parameter.

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