drm/ttm: Add helpers for shrinking

Add a number of helpers for shrinking that access core TTM and
core MM functionality in a way that make them unsuitable for
driver open-coding.

v11:
- New patch (split off from previous) and additional helpers.
v13:
- Adapt to ttm_backup interface change.
- Take resource off LRU when backed up.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/intel-xe/20250305092220.123405-6-thomas.hellstrom@linux.intel.com