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: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel 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;