drm/i915: fix page flip finish vs. prepare on plane B

The refreshed patch had a copy & paste bug.

Reported-by: Simon Farnsworth <simon.farnsworth@onelan.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>