drm/i915: Treat an error from i915_vma_instance() as unlikely

When pinning into the global GTT, an error from creating the VMA is
unlikely, so mark it so.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170119192659.31789-4-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>