drm/ttm: fix incorrect calculate on shrink_pages

shrink_pages is in unit of Order after ttm_page_pool_free,
but it is used by nr_free in next round so need change
it into native page unit

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Roger He <Hongbo.He@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>