drm/ttm: remove io_reserve_fastpath flag

Just use the use_io_reserve_lru flag. It doesn't make much
sense to have two flags.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/378238/