drm/i915: Fix idle_frames counter.

'commit 97173eaf5 ("drm/i915: PSR: Increase idle_frames")' was a mistake.
The special case it tried to cover was already being covered by
the DP_PSR_NO_TRAIN_ON_EXIT. So this ended up duplicated.

So, instead of reverting that let's take this opportunity and unify
the idle_frame definition in a single place so we standardize the access
and avoid room for that same mistake again.

Few changes with this patch:

1. Instead of just respecting the VBT we set a
global minumum with max(). So we are sure that we will avoid corner cases
in case VBT is doing something we don't understand.

2. Instead of minimum 5 we use 6. When introducing the idle_frames += 4 case
we considered that minimum was 2. All because the off-by-one issue.

v2: Unified idle_frame definition.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449528320-27655-1-git-send-email-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_psr.c

@@ -267,23 +267,17 @@ static void hsw_psr_enable_source(struct intel_dp *intel_dp)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
 	uint32_t max_sleep_time = 0x1f;
-	/* Lately it was identified that depending on panel idle frame count
-	 * calculated at HW can be off by 1. So let's use what came
-	 * from VBT + 1.
-	 * There are also other cases where panel demands at least 4
-	 * but VBT is not being set. To cover these 2 cases lets use
-	 * at least 5 when VBT isn't set to be on the safest side.
+	/*
+	 * Let's respect VBT in case VBT asks a higher idle_frame value.
+	 * Let's use 6 as the minimum to cover all known cases including
+	 * the off-by-one issue that HW has in some cases. Also there are
+	 * cases where sink should be able to train
+	 * with the 5 or 6 idle patterns.
 	 */
-	uint32_t idle_frames = dev_priv->vbt.psr.idle_frames ?
-			       dev_priv->vbt.psr.idle_frames + 1 : 5;
+	uint32_t idle_frames = max(6, dev_priv->vbt.psr.idle_frames);
 	uint32_t val = 0x0;
 	const uint32_t link_entry_time = EDP_PSR_MIN_LINK_ENTRY_TIME_8_LINES;
 
-	if (intel_dp->psr_dpcd[1] & DP_PSR_NO_TRAIN_ON_EXIT) {
-		/* Sink should be able to train with the 5 or 6 idle patterns */
-		idle_frames += 4;
-	}
-
 	I915_WRITE(EDP_PSR_CTL, val |
 		   (IS_BROADWELL(dev) ? 0 : link_entry_time) |
 		   max_sleep_time << EDP_PSR_MAX_SLEEP_TIME_SHIFT |