drm/i915: make i915_suspend_switcheroo static

Looks like this was missed when unexporting, so let's keep sparse happy.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 7f26cb88 ("drm/i915: Unexport VGA switcheroo functions")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161202102411.19831-1-matthew.auld@intel.com