drm/amdgpu: add VM support for huge pages v2

Convert GTT mappings into linear ones for huge page handling.

v2: use fragment size as minimum for linear conversion

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