drm/i915: Move the release of PT page to the upper caller

a PT page will be released if it doesn't contain any meaningful mappings
during PPGTT page table shrinking. The PT entry in the upper level will
be set to a scratch entry.

Normally this works nicely, but in virtualization world, the PPGTT page
table is tracked by hypervisor. Releasing the PT page before modifying
the upper level PT entry would cause extra efforts.

As the tracked page has been returned to OS before losing track from
hypervisor, it could be written in any pattern. Hypervisor has to recognize
if a page is still being used as a PT page by validating these writing
patterns. It's complicated. Better let the guest modify the PT entry in
upper level PT first, then release the PT page.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhiyuan Lv <zhiyuan.lv@intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Link: https://patchwork.freedesktop.org/patch/122697/msgid/1479728666-25333-1-git-send-email-zhi.a.wang@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1480402516-22275-1-git-send-email-zhi.a.wang@intel.com

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