drm/i915: don't disable a PCH DPLL that's in use

If a PCH pipe PLL is being used by transcoder C, don't disable it.
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/intel_display.c

@@ -1204,7 +1204,8 @@ static void intel_disable_pch_pll(struct drm_i915_private *dev_priv,
 				  enum pipe pipe)
 {
 	int reg;
-	u32 val;
+	u32 val, pll_mask = TRANSC_DPLL_ENABLE | TRANSC_DPLLB_SEL,
+		pll_sel = TRANSC_DPLL_ENABLE;
 
 	if (pipe > 1)
 		return;
@@ -1215,6 +1216,15 @@ static void intel_disable_pch_pll(struct drm_i915_private *dev_priv,
 	/* Make sure transcoder isn't still depending on us */
 	assert_transcoder_disabled(dev_priv, pipe);
 
+	if (pipe == 0)
+		pll_sel |= TRANSC_DPLLA_SEL;
+	else if (pipe == 1)
+		pll_sel |= TRANSC_DPLLB_SEL;
+
+
+	if ((I915_READ(PCH_DPLL_SEL) & pll_mask) == pll_sel)
+		return;
+
 	reg = PCH_DPLL(pipe);
 	val = I915_READ(reg);
 	val &= ~DPLL_VCO_ENABLE;