frameworks/native: fix copy back dirty region

Copyback dirty region logic does copyback,
even when its not necessary causing 2ms delay.
Fix the logic to copy back only what is necessary.

Change-Id: I52de68258ac9f87d704ee5401f93417805fa6773
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_frameworks_native.git into cm-11.0