drm/i915: Ensure that if we ever try to pin+fence it is mappable.

When merging Daniel's full-gtt patches I had a set of tweaks which I
thought I had undone. I was half right...

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31286


Reported-by:  <jinjin.wang@intel.com>
Reported-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>