drm/amdgpu: Fix huge page updates with CPU

Correctly detect system memory mappings when using CPU and don't use
huge pages for them.

Avoid incorrectly translating a physical page table GPU address when
splitting a huge page while mapping system memory.

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>