drm/i915/ddi: add break in DDI mode select switch

The lack of a break here wasn't for falling through to some other
important code, so made me do a double take.  Add a break just to make
things a little less confusing.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>