drm/i915/vlv: Add cdclk workaround for DSI

At least on the Chuwi Vi8 (non pro/plus) the LCD panel will show an image
shifted aprox. 20% to the left (with wraparound) and sometimes also wrong
colors, showing that the panel controller is starting with sampling the
datastream somewhere mid-line. This happens after the first blanking and
re-init of the panel.

After looking at drm.debug output I noticed that initially we inherit the
cdclk of 333333 KHz set by the GOP, but after the re-init we picked 266667
KHz, which turns out to be the cause of this problem, a quick hack to hard
code the cdclk to 333333 KHz makes the problem go away.

I've tested this on various Bay Trail devices, to make sure this not does
cause regressions on other devices and the higher cdclk does not cause
any problems on the following devices:
-GP-electronic T701      1024x600   333333 KHz cdclk after this patch
-PEAQ C1010              1920x1200  333333 KHz cdclk after this patch
-PoV mobii-wintab-800w    800x1280  333333 KHz cdclk after this patch
-Asus Transformer-T100TA 1368x768   320000 KHz cdclk after this patch

Also interesting wrt this is the comment in vlv_calc_cdclk about the
existing workaround to avoid 200 Mhz as clock because that causes issues
in some cases.

This commit extends the "do not use 200 Mhz" workaround with an extra
check to require atleast 320000 KHz (avoiding 266667 KHz) when a DSI
panel is active.

Changes in v2:
-Change the commit message and the code comment to not treat the GOP as
 a reference, the GOP should not be treated as a reference
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171220105017.11259-1-hdegoede@redhat.com

drivers/gpu/drm/i915/intel_cdclk.c

@@ -1940,6 +1940,14 @@ int intel_crtc_compute_min_cdclk(const struct intel_crtc_state *crtc_state)
 	if (crtc_state->has_audio && INTEL_GEN(dev_priv) >= 9)
 		min_cdclk = max(2 * 96000, min_cdclk);
 
+	/*
+	 * On Valleyview some DSI panels lose (v|h)sync when the clock is lower
+	 * than 320000KHz.
+	 */
+	if (intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DSI) &&
+	    IS_VALLEYVIEW(dev_priv))
+		min_cdclk = max(320000, 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);