drm/i915: eliminate dead domain clearing on reset

The code itself is no longer accurate without updating once we have
multiple address space since clearing the domains of every object
requires scanning the inactive list for all VMs.

"This code is dead. Just remove it rather than port it to vma." - Chris
Wilson

Recommended-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>