libvpx

Fix int64_t to unsigned int conversion warnings

Use unsigned int type to store the sse in the pixel domain. The
precision is sufficient to handle sse of block size up to 64x64.
The transform domain version however needs int64_t, since there is
a transfer gain applied in the forward transformation that might
cause unsigned int overflow.

Change-Id: Ifef97c38597e426262290f35341fbb093cf0a079