drm/i915: Drop redundant GGTT rebinding

Since

commit bf3d149b
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Feb 14 14:01:12 2014 +0100

    drm/i915: split PIN_GLOBAL out from PIN_MAPPABLE

i915_gem_obj_ggtt_pin always binds into the ggtt, but I've forgotten
to remove the now redundant additional bind call later on. Fix this
up.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>