drm/i915: Consolidate i915_vma_unpin_and_release()

In a few places, we repeat a call to clear a pointer to a vma whilst
unpinning and releasing a reference to its owner. Refactor those into a
common function.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-26-git-send-email-chris@chris-wilson.co.uk