drm/i915: Remove inactive LRU tracking from set_domain_ioctl

As the userspace mappings are torn down on every GPU write, we prefer to
track when the buffer is activated (via a fresh i915_gem_fault). This
makes the LRU conceptually simpler. With coherent mappings, the
remaining use-case for set_domain_ioctl is GPU synchronisation.

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