Revert "drm/ttm: make ttm_bo_unpin more defensive"

This reverts commit 4148f28f which is
commit 6c540317 upstream.

It breaks the Android kernel abi, and is not needed for any Android
devices at this time.  If it is needed in the future, it can be brought
back in an abi-safe way.

Bug: 161946584
Change-Id: I583e85294adb0b0110b0da28b830001a002cc946
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>