drm/ttm: fix ttm_bo_swapout

Commit 7120a447 ("drm/ttm: Double check mem_type of BO while eviction")
made ttm_bo_evict_swapout_allowable() function actually check the
placement, but we always used a dummy placement in ttm_bo_swapout.

Fix this by using the real placement instead.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 7120a447 ("drm/ttm: Double check mem_type of BO while eviction")
Reviewed-by: Pan, Xinhui <Xinhui.Pan@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211202103828.44573-1-christian.koenig@amd.com