Make handling of dev_priv->vblank_pipe more robust.

Initialize it to default value if it hasn't been set by the X server yet.

In i915_vblank_pipe_set(), only update dev_priv->vblank_pipe and call
i915_enable_interrupt() if the argument passed from userspace is valid to avoid
corrupting dev_priv->vblank_pipe on invalid arguments.
(cherry picked from 87c57cba commit)