gralloc: Fix NV12 and NV21 stride

NV12 spec says that the chroma stride is the same as the luma
stride. The camera hardware produces YUV data per this spec.
NV21 has the same behaviour with the chroma samples reversed.
Venus also conforms to this.

Change-Id: I6e107b2a3ba6f51e135348bd147a9c26be003bb2
1 file changed