drm/i915/ttm: add some sanity checks for lmem_userfault_list

drm/i915/ttm: add some sanity checks for lmem_userfault_list

Rather than getting some hard to debug uaf, add some warns to hopefully
catch issues with userfault_count being non-zero when destroying the
object. Also if we somehow add an object to lmem_userfault_list that
somehow doesn't map lmem.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/7469
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221107165414.56970-2-matthew.auld@intel.com