drm/i915: Remove unused dev_priv->vblank_pipe

vblank_pipe was intended to be used for tracking DRI1 state. However,
the vblank_pipe reported to DRI1 is fixed to umask both pipes, and the
dev_priv->vblank_pipe unused and superfluous.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>