set chroma width/height correctly for 422/444 in dav1d_read_coef_blocks

Fixes unaligned writes while splatting coefs for skip blocks with
clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5684725352497152 and
clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5728508249112576.
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