drm/i915/ttm: add unmap_virtual callback

Ensure we call ttm_bo_unmap_virtual when releasing the pages.
Importantly this should now handle the ttm swapping case, and all other
places that already call into i915_ttm_move_notify().

v2: fix up the selftest

Fixes: cf3e3e86

 ("drm/i915: Use ttm mmap handling for ttm bo's.")
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/20220106174910.280616-3-matthew.auld@intel.com