intrabc: adjust right border to multiple of the block size

This fixes an overlapping memcpy with
clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5734069753610240. This is
a regression introduced by "intrabc: use visible width/height in mv
correction".

This reverts commit 7677c120.
14 jobs for master in 5 minutes and 9 seconds (queued for 1 second)
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