drm/ttm: Fix trace include path (v2)

Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

(v2): Drop Makefile change too.