drm/ttm: Avoid NULL pointer deref for evicted BOs

drm/ttm: Avoid NULL pointer deref for evicted BOs

It is possible for a BO to exist that is not currently associated with a
resource, e.g. because it has been evicted.

When devcoredump tries to read the contents of all BOs for dumping, we need
to expect this as well -- in this case, ENODATA is recorded instead of the
buffer contents.

Fixes: 7d08df5d0bd3 ("drm/ttm: Add ttm_bo_access")
Fixes: 09ac4fcb3f25 ("drm/ttm: Implement vm_operations_struct.access v2")
Cc: stable <stable@kernel.org>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6271
Signed-off-by: Simon Richter <Simon.Richter@hogyros.de>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251013161241.709916-1-Simon.Richter@hogyros.de