drm/i915: Only discard backing storage on releasing the last ref

Before purging our pages (as opposed to copying back the contents from
the GPU), make sure that there is not an exposed CPU mmapping through
which the user can inspect the results.

Regression from

commit 5537252b
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Mar 25 13:23:06 2014 +0000

    drm/i915: Invalidate our pages under memory pressure

Testcase: igt/gem_mmap/new-object
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79005


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Guo Jinxian <jinxianx.guo@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>