Revert "drm/ttm: check null pointer before accessing when swapping"

This reverts commit d39971d9 which is
commit 2dedcf41 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: I3df03d40d2e60351b9fee2ba30b5cd3dcd1cf35f
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>