drm/i915: Create a global list of vms

After we plumb our code to support multiple address spaces (VMs), there
are a few situations where we want to be able to traverse the list of
all address spaces in the system. Cases like eviction, or error state
collection are obvious example.

v2: Delete the global link instead of the list head. While this in and
of itself shouldn't be really be a problem, doing this allows us to WARN
on an non-empty list, which is a problem. (Daniel)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>