drm/i915: Remove defunct i915->vm_list

No longer used and can be removed. One less global that currently
demands struct_mutex protection.

References: e9e7dc41

 ("drm/i915/gtt: Make gen6 page directories evictable")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705065653.20449-1-chris@chris-wilson.co.uk