In i915_gem_object_pin obviously unbind only if mappable is true.

This is the last part to enable gtt_mappable_end != gtt_size, which
the next patch will do.

v2: Fences on g33/pineview only work in the mappable part of the
gtt.
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>