drm/i915: don't set unpin_work if vblank_get fails

This fixes a race where we may try to finish a page flip and decrement
the refcount even if our vblank_get failed and we ended up with a
spurious flip pending interrupt.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=34211

.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>