aarch64: vp9itxfm: Use w3 instead of x3 for the int eob parameter

The clobbering tests in checkasm are only invoked when testing
correctness, so this bug didn't show up when benchmarking the
dc-only version.

This is cherrypicked from libav commit
4d960a11855f4212eb3a4e470ce890db7f01df29.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
