drm/i915: Power down the DSI PLL before reconfiguring it

On VLV at least, the BIOS may leave the DSI PLL enabled in some wonky
state where it just refuses to lock. Simply disabling the PLL before
reconfiguring it is not enough to fix it, but power gating the PLL
prior to reconfiguring does work.

This happens on BYT FFRD8 when booting with HDMI connected so the DSI
display will not be lit up by the BIOS.

Also we can remove the code for BXT that disables the PLL before
enabling it again.

v2: s/vlv/intel/ since BXT made thing generic
v3: Remove the BXT disable PLL before enable trick

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458052809-23426-11-git-send-email-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_dsi.c

@@ -505,7 +505,13 @@ static void intel_dsi_pre_enable(struct intel_encoder *encoder)
 
 	DRM_DEBUG_KMS("\n");
 
+	/*
+	 * The BIOS may leave the PLL in a wonky state where it doesn't
+	 * lock. It needs to be fully powered down to fix it.
+	 */
+	intel_disable_dsi_pll(encoder);
 	intel_enable_dsi_pll(encoder);
+
 	intel_dsi_prepare(encoder);
 
 	/* Panel Enable over CRC PMIC */

drivers/gpu/drm/i915/intel_dsi_pll.c

@@ -484,14 +484,6 @@ static void bxt_enable_dsi_pll(struct intel_encoder *encoder)
 
 	DRM_DEBUG_KMS("\n");
 
-	val = I915_READ(BXT_DSI_PLL_ENABLE);
-
-	if (val & BXT_DSI_PLL_DO_ENABLE) {
-		WARN(1, "DSI PLL already enabled. Disabling it.\n");
-		val &= ~BXT_DSI_PLL_DO_ENABLE;
-		I915_WRITE(BXT_DSI_PLL_ENABLE, val);
-	}
-
 	/* Configure PLL vales */
 	if (!bxt_configure_dsi_pll(encoder)) {
 		DRM_ERROR("Configure DSI PLL failed, abort PLL enable\n");