drm/i915/ttm: only fault WILLNEED objects

Don't attempt to fault and re-populate purged objects. By some fluke
this passes the dontneed-after-mmap IGT, but for the wrong reasons.

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/20220106174910.280616-2-matthew.auld@intel.com