drm/i915: Simplify VLV drain latency computation

The current drain lantency computation relies on hardcoded limits to
determine when the to use the low vs. high precision multiplier.
Rewrite the code to use a more straightforward approach.
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -755,12 +755,15 @@ static bool vlv_compute_drain_latency(struct drm_crtc *crtc,
 		return false;
 
 	entries = DIV_ROUND_UP(clock, 1000) * pixel_size;
-	if (IS_CHERRYVIEW(dev))
-		*prec_mult = (entries > 32) ? 16 : 8;
-	else
-		*prec_mult = (entries > 128) ? 64 : 32;
+
+	*prec_mult = IS_CHERRYVIEW(dev) ? 16 : 64;
 	*drain_latency = (64 * (*prec_mult) * 4) / entries;
 
+	if (*drain_latency > DRAIN_LATENCY_MASK) {
+		*prec_mult /= 2;
+		*drain_latency = (64 * (*prec_mult) * 4) / entries;
+	}
+
 	if (*drain_latency > DRAIN_LATENCY_MASK)
 		*drain_latency = DRAIN_LATENCY_MASK;