hwui: Don't discard framebuffer on exynos4

The layers gets empty/not cleaned and redrawn on top of the preview,
causing beautiful glitch on homescreen, and drawing black instead
of transparency.

Change-Id: I633b104224c73c625f1193ee3dde41823cddc5bf
Merge branch 'cm-10.2' of git://github.com/CyanogenMod/android_frameworks_base into cm-10.2