drm/amdgpu: use amdgpu_bo_user bo for metadata and tiling flag

Tiling flag and metadata are only needed for BOs created by
amdgpu_gem_object_create(), so we can remove those from the
base class.

v2: * squash tiling_flags and metadata relared patches into one
    * use BUG_ON for non ttm_bo_type_device type when accessing
    tiling_flags and metadata._
v3: *include to_amdgpu_bo_user

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>