drm/i915/gem: Fix gem_madvise for ttm+shmem objects

Gem-TTM objects that are backed by shmem might have populated
page-vectors without having the GEM pages set. Those objects
aren't moved to the correct shrinker / purge list by gem_madvise.

For such objects, identified by having the
_SELF_MANAGED_SHRINK_LIST set, make sure they end up on the
correct list.

v2:
- Revert a change that made swapped-out objects inaccessible for
  truncating. (Matthew Auld)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211108123637.929617-1-thomas.hellstrom@linux.intel.com