drm/i915: Micro-optimise i915_get_ggtt_vma_pages()

The predominant VMA class is normal GTT, so allow gcc to emphasize that
path and avoid unnecessary stack movement.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-1-chris@chris-wilson.co.uk