drm/i915/vlv: Fix typo in the DPIO register define.

Incorrect definition DPIO_TX3_SWING_CTL4.

From Ville's review: "Based on the specs, the typo meant that HDMI B
ended up using "incorrect" de-emphasis for the TMDS data lanes."

Signed-off-by: Chon Ming Lee <chon.ming.lee@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Add comment from Ville's review about the impact.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>