drm/i915/ttm: Fix memory leaks

Fix two memory leaks introduced with the ttm backend.

Fixes: 213d5092 ("drm/i915/ttm: Introduce a TTM i915 gem object backend")
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210615122408.32347-1-thomas.hellstrom@linux.intel.com