Kindle: Disable HW dithering on Scribe

Pending the actual fix from
https://github.com/koreader/koreader-base/pull/1718 that'll hit after
the release.

frontend/device/kindle/device.lua

@@ -639,7 +639,7 @@ local KindleScribe = Kindle:extend{
     hasGSensor = yes,
     display_dpi = 300,
     touch_dev = "/dev/input/touch",
-    canHWDither = yes,
+    canHWDither = no,
     canDoSwipeAnimation = yes,
 }