drm/ttm: clean up non-x86 definitions on ttm_tt

All non-x86 definitions are moved to ttm_set_memory header, so remove it from
ttm_tt.c.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>