drm/i915: Nuke MI_ARB_STATE save/restore

Originally added in commit 1f84e550 ("drm/i915 more registers for
S3 (DSPCLK_GATE_D, CACHE_MODE_0, MI_ARB_STATE)") to fix some underruns.
I suspect that was due to the trickle feed settings getting clobbered
during suspend. We've been disabling trickle feed explicitly since
commit 20f94967 ("drm/i915: Disable trickle feed via MI_ARB_STATE
for the gen4") so this magic save/restore should no longer be needed.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200908140210.31048-3-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -538,7 +538,6 @@ struct intel_gmbus {
 struct i915_suspend_saved_registers {
 	u32 saveDSPARB;
 	u32 saveCACHE_MODE_0;
-	u32 saveMI_ARB_STATE;
 	u32 saveSWF0[16];
 	u32 saveSWF1[16];
 	u32 saveSWF3[3];

drivers/gpu/drm/i915/i915_suspend.c

@@ -75,9 +75,6 @@ int i915_save_state(struct drm_i915_private *dev_priv)
 	if (INTEL_GEN(dev_priv) < 7)
 		dev_priv->regfile.saveCACHE_MODE_0 = I915_READ(CACHE_MODE_0);
 
-	/* Memory Arbitration state */
-	dev_priv->regfile.saveMI_ARB_STATE = I915_READ(MI_ARB_STATE);
-
 	/* Scratch space */
 	if (IS_GEN(dev_priv, 2) && IS_MOBILE(dev_priv)) {
 		for (i = 0; i < 7; i++) {
@@ -112,9 +109,6 @@ int i915_restore_state(struct drm_i915_private *dev_priv)
 		I915_WRITE(CACHE_MODE_0, dev_priv->regfile.saveCACHE_MODE_0 |
 			   0xffff0000);
 
-	/* Memory arbitration state */
-	I915_WRITE(MI_ARB_STATE, dev_priv->regfile.saveMI_ARB_STATE | 0xffff0000);
-
 	/* Scratch space */
 	if (IS_GEN(dev_priv, 2) && IS_MOBILE(dev_priv)) {
 		for (i = 0; i < 7; i++) {