drm/i915: set CDCLK if DPLL0 enabled during resuming from S3

Since BIOS RC 1.4 it would enable CDCLK PLL during BIOS S3 resume, then
driver needs to set CDCLK to avoid display corruption if DPLL0 enabled.

References: https://bugs.freedesktop.org/show_bug.cgi?id=91697Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Cooper Chiou <cooper.chiou@intel.com>
Reviewed-by: Wei Shun Chang <wei.shun.chang@intel.com>
Tested-by: Gary Wang <gary.c.wang@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Gavin Hindman <gavin.hindman@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Xiong Y Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Gary Wang <gary.c.wang@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -5712,16 +5712,13 @@ void skl_init_cdclk(struct drm_i915_private *dev_priv)
 	/* enable PG1 and Misc I/O */
 	intel_display_power_get(dev_priv, POWER_DOMAIN_PLLS);
 
-	/* DPLL0 already enabed !? */
-	if (I915_READ(LCPLL1_CTL) & LCPLL_PLL_ENABLE) {
-		DRM_DEBUG_DRIVER("DPLL0 already running\n");
-		return;
+	/* DPLL0 not enabled (happens on early BIOS versions) */
+	if (!(I915_READ(LCPLL1_CTL) & LCPLL_PLL_ENABLE)) {
+		/* enable DPLL0 */
+		required_vco = skl_cdclk_get_vco(dev_priv->skl_boot_cdclk);
+		skl_dpll0_enable(dev_priv, required_vco);
 	}
 
-	/* enable DPLL0 */
-	required_vco = skl_cdclk_get_vco(dev_priv->skl_boot_cdclk);
-	skl_dpll0_enable(dev_priv, required_vco);
-
 	/* set CDCLK to the frequency the BIOS chose */
 	skl_set_cdclk(dev_priv, dev_priv->skl_boot_cdclk);