If we fail to allocate the ppgtt range after allocating the pages for
the vma, we should unwind the local allocation before reporting back the
failure.

Fixes: ff685975 ("drm/i915: Move allocate_va_range to GTT")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227122654.27651-2-chris@chris-wilson.co.uk