drm/amdgpu: fb BO should be ttm_bo_type_device

FB BO should not be ttm_bo_type_kernel type and
amdgpufb_create_pinned_object() pins the FB BO anyway.

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