drm/i915/skl: Don't warn if reading back DPLL0 is disabled

We can operate with DPLL0 off with CDCLK backed by the 24Mhz reference
clock, and that's a supported configuration. Don't warn when notice
DPLL0 is off then.

We still have a separate warn at boot if cdclk is disabled (because we
don't currently try to handle the case (that shouldn't happen on SKL as
far as I know) where we boot with display not initialized.
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -6737,10 +6737,8 @@ static int skylake_get_display_clock_speed(struct drm_device *dev)
 	uint32_t cdctl = I915_READ(CDCLK_CTL);
 	uint32_t linkrate;
 
-	if (!(lcpll1 & LCPLL_PLL_ENABLE)) {
-		WARN(1, "LCPLL1 not enabled\n");
+	if (!(lcpll1 & LCPLL_PLL_ENABLE))
 		return 24000; /* 24MHz is the cd freq with NSSC ref */
-	}
 
 	if ((cdctl & CDCLK_FREQ_SEL_MASK) == CDCLK_FREQ_540)
 		return 540000;