The physical object is treated as permanently pinned. If we fail to take
this initial pin during i915_gem_object_attach_phys() we need to revert
it back to an ordinary shmemfs object before reporting the failure.

v2: git-add
Reported-by: NMika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215163900.11606-1-chris@chris-wilson.co.ukReviewed-by: NMika Kuoppala <mika.kuoppala@intel.com>