drm/i915: Add "missing" break to haswell_get_ddi_pll()

While not technically needed on the last case in the switch statement,
the 'break' makes it look better IMO.

v2: Fixed a typo in the commit message (Paulo)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449005527-15617-1-git-send-email-ville.syrjala@linux.intel.com