drm/ttm: remove stale declaration and field

Patch 649bf3ca has completely
removed ttm_backend structure. Remove lingering declaration
and related (now stale) field in ttm_tt structure,

CC: Jerome Glisse <jglisse at redhat.com>
Signed-off-by: Ilija Hadzic <ihadzic at research.bell-labs.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>