drm/i915/ttm: ensure we unmap when purging

Purging can happen during swapping out, or directly invoked with the
madvise ioctl. In such cases this doesn't involve a ttm move, which
skips umapping the object.

v2(Thomas):
- add ttm_truncate helper, and just call into i915_ttm_move_notify() to
  handle the unmapping step

Fixes: 213d5092

 ("drm/i915/ttm: Introduce a TTM i915 gem object backend")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220106174910.280616-4-matthew.auld@intel.com