In the near future TTM will have NULL bo->resource when the object is
initially created, plus after calling into pipeline-gutting. Try to
handle the remaining cases. In practice NULL bo->resource should be
taken to mean swapped-out or purged object.

v2 (Andrzej):
  - Rather make i915_ttm_cpu_maps_iomem() return false with NULL
    resource.

References: 516198d3 ("drm/i915: audit bo->resource usage v3")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230130101230.25347-2-matthew.auld@intel.comReviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Acked-by: Christian König <ckoenig.leichtzumerken@gmail.com>
Signed-off-by: Christian König <christian.koenig@amd.com>