drm/i915: use pipes, not planes to label vblank data

vblank in the kernel is far simpler if it deals with pipes instead of
planes, so we're changing both user and kernel side.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>