drm/i915/ttm: remove the virtualized start hack

This was mostly needed to differentiate between mappable and
non-mappable lmem, such that ttm would understand non-mappable ->
mappable moves (or vice versa), and not just turn them into noops. We
have since gained proper .intersects() and .compatible() hooks for the
resource manager, which takes care of this for us.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221220112736.161642-1-matthew.auld@intel.com