gralloc: fix getYUVPlaneInfo for YV12 format

- Use proper alignment for cstride while calculating the
  ycbcr info for YV12 format
- This fixes the corruption issue when the video is composed
  using GPU

CRs-fixed: 711696
Change-Id: I776cc6c5ce781ea3de25b5d8427345b512a8305a