drm/ttm: Add ttm_set_pages_wc and ttm_set_pages_uc helper

These two helpers will be used on set page caching.

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>