drm/i915: Try harder to disable trickle feed on VLV

The specs are a bit unclear whether the per-plane trickle feed disable
control exists on VLV. There is another trickle feed disable control
in the MI_ARB register.

After some experimentation it turns out both the DSPCNTR trickle feed
bits and the MI_ARB bit can be toggled. However the DSPCNTR bits don't
seem to have any effect.

The MI_ARB bit, on the other hand, has a noticable effect. I performed
an experiment where I reduced the FIFO size via DSPARB and observed the
effect of the MI_ARB trickle feed bit on the display.

Using a 1920x1080-60 mode, with MI_ARB=0x4 the display started to have
problems with DSPARB=0x42424242, whereas with MI_ARB=0x0 the problems
didn't start until DSPARB=0x09090909. This seems to confirm that the
MI_ARB trickle feed bit actually does work.

So replace the use of the DSPCNTR trickle feed bits with MI_ARB
on VLV.

v2: Amend commit message with results from experimentation

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -1367,6 +1367,8 @@
 #define FW_BLC_SELF_VLV		(VLV_DISPLAY_BASE + 0x6500)
 #define  FW_CSPWRDWNEN		(1<<15)
 
+#define MI_ARB_VLV		(VLV_DISPLAY_BASE + 0x6504)
+
 /*
  * Palette regs
  */

drivers/gpu/drm/i915/intel_pm.c

@@ -4873,7 +4873,7 @@ static void valleyview_init_clock_gating(struct drm_device *dev)
 
 	I915_WRITE(GEN7_UCGCTL4, GEN7_L3BANK2X_CLOCK_GATE_DISABLE);
 
-	g4x_disable_trickle_feed(dev);
+	I915_WRITE(MI_ARB_VLV, MI_ARB_DISPLAY_TRICKLE_FEED_DISABLE);
 
 	I915_WRITE(CACHE_MODE_1,
 		   _MASKED_BIT_ENABLE(PIXEL_SUBSPAN_COLLECT_OPT_DISABLE));