surfaceflinger: Enable dithering if persist.sys.use_dithering=1|2

* Fixes color banding(1) and blur effect(2)

0: surface doesn't need dithering
1: enable dithering if necessary (default)
2: enable dithering (always)

Change-Id: Icda52b0b11ed3e2fa0db769d581582c23beb90fd