drm/i915: Corrupt DSI picture fix for GeminiLake

Currently due to regression CI machine displays show corrupt picture.
Problem is when CDCLK is as low as 79200, picture gets unstable, while
DSI and DE pll values were confirmed to be correct.  Limiting to 158400
as agreed with Ville.

We could not come up with any better solution yet, as PLL divider values
both for MIPI(DSI PLL) and CDCLK(DE PLL) are correct, however seems that
due to some boundary conditions, when clocking is too low we get wrong
timings for DSI display.  Similar workaround exists for VLV though, so
just took similar condition into use. At least that way GLK platform
will start to be usable again, with current drm-tip.

v2: Fixed commit subject as suggested.

v3: Added generic bugs(crc failures, screen not init
for GLK DSI which might be affected).

v4: Added references tag for bugs affected.
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=109267
References: https://bugs.freedesktop.org/show_bug.cgi?id=103184Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190430125119.7478-1-stanislav.lisovskiy@intel.com

drivers/gpu/drm/i915/intel_cdclk.c

@@ -2265,6 +2265,15 @@ int intel_crtc_compute_min_cdclk(const struct intel_crtc_state *crtc_state)
 	    IS_VALLEYVIEW(dev_priv))
 		min_cdclk = max(320000, min_cdclk);
 
+	/*
+	 * On Geminilake once the CDCLK gets as low as 79200
+	 * picture gets unstable, despite that values are
+	 * correct for DSI PLL and DE PLL.
+	 */
+	if (intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DSI) &&
+	    IS_GEMINILAKE(dev_priv))
+		min_cdclk = max(158400, min_cdclk);
+
 	if (min_cdclk > dev_priv->max_cdclk_freq) {
 		DRM_DEBUG_KMS("required cdclk (%d kHz) exceeds max (%d kHz)\n",
 			      min_cdclk, dev_priv->max_cdclk_freq);