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_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index b5b6d19..399b181 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -689,6 +689,8 @@
 				     struct drm_file *file_priv);
 
 extern u32 intel_dpio_read(struct drm_i915_private *dev_priv, int reg);
+extern void intel_dpio_write(struct drm_i915_private *dev_priv, int reg,
+			     u32 val);
 
 /* Power-related functions, located in intel_pm.c */
 extern void intel_init_pm(struct drm_device *dev);