intel: Reset vma list upon purge

During free we unconditionally delete the bo from the vma cache. This
relies on the its list member being kept in a sane state. This fails
after the object is purged, as the purge operation performs a pure
deletion and doesn't reset the list member, leaving a pair of dangling
pointers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>