drm/ttm: add new function to check if bo is allowable to evict or swapout

extract a function as ttm_bo_evict_swapout_allowable since eviction and
swapout can share same logic.

v2: modify commit message and add description in the code

Reviewed-by: Thomas Hellström <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chuming Zhou <david1.zhou@amd.com>
Signed-off-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>