drm/i915/gtt: Free unused page tables on unbind the context

As we cannot reliably change used page tables while the context is
active, the earliest opportunity we have to recover excess pages is when
the context becomes idle. So whenever we unbind the context (it must be
idle, and indeed being evicted) free the unused ptes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180614134315.5900-2-chris@chris-wilson.co.uk