Correct the plane naming within the D3D code

Within I420 (as the decoder outputs), the first chroma plane is
U and the second is V, and similarly, in NV12, the chroma components
are written in the order U, V.

This doesn't have any practical effect, it only makes the
variable naming while it previously was misleading.
Merge pull request #280 from mstorsjo/d3d-plane-naming

Correct the plane naming within the D3D code