drm/i915/dp: program VSwing and Preemphasis control settings on VLV v2

Program few Tx buffer Swing control settings through DPIO.

v2: fix up codingstyle (Daniel)
    call from set_signal_levels (Ville, Daniel)
    use proper port numbers (Jesse)

Signed-off-by: Pallavi G <pallavi.g@intel.com>
Signed-off-by: Yogesh M <yogesh.mohan.marimuthu@intel.com>
Signed-off-by: Gajanan Bhat <gajanan.bhat@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org> (v2 changes)
[danvet: Reorder if-ladder to avoid two IS_VLV checks.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index bdbad76..9ff992a 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -450,8 +450,7 @@
 	return I915_READ(DPIO_DATA);
 }
 
-static void intel_dpio_write(struct drm_i915_private *dev_priv, int reg,
-			     u32 val)
+void intel_dpio_write(struct drm_i915_private *dev_priv, int reg, u32 val)
 {
 	WARN_ON(!mutex_is_locked(&dev_priv->dpio_lock));