drm/i915/bxt: Reversed polarity of PORT_PLL_REF_SEL bit

For BXT, description of polarities of PORT_PLL_REF_SEL
has been reversed for newer Gen9LP steppings according to the
recent update in Bspec. This bit now should be set for
"Non-SSC" mode for all Gen9LP starting from B0 stepping.

v2: Only B0 and newer stepping should be affected by this
change.
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94866Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458176773-26925-1-git-send-email-dongwon.kim@intel.com

drivers/gpu/drm/i915/intel_dpll_mgr.c

@@ -1296,7 +1296,15 @@ static void bxt_ddi_pll_enable(struct drm_i915_private *dev_priv,
 	enum port port = (enum port)pll->id;	/* 1:1 port->PLL mapping */
 
 	temp = I915_READ(BXT_PORT_PLL_ENABLE(port));
+	/*
+	 * Definition of each bit polarity has been changed
+	 * after A1 stepping
+	 */
+	if (IS_BXT_REVID(dev_priv, 0, BXT_REVID_A1))
 		temp &= ~PORT_PLL_REF_SEL;
+	else
+		temp |= PORT_PLL_REF_SEL;
+
 	/* Non-SSC reference */
 	I915_WRITE(BXT_PORT_PLL_ENABLE(port), temp);