fixed a typo that mis-used Y plane stride for UV blocks.

Raised by Lei Yang, the Y plane stride was used for UV blocks.
This is clearly a typo. But as the comments in the code suggested
that this port of code has not been used yet, so the typo should
not have created any damage yet.

Change-Id: Iea895edc17469a51c803a8cc6d0fce65a1a7fc2f