drm/i915: Remove pm park/unpark notifications

With the last user, i915_vma_parked(), retired, there are no more users
of the per-gt pm notifications and we can remove the unused
infrastructure.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021183236.21790-2-chris@chris-wilson.co.uk