Fix dtype-argument bug in dask_cudf read_csv

[rjzamora]

Closes #9719

dask_cudf.read_csv currently fails when both usecols and dtype are specified. This PR is a simple fix. In the near future, the _internal_read_csv implementation should also be modified to produce a Blockwise HLG Layer, but I will leave that for a separate PR.

[codecov]

Codecov Report

Merging #9796 (a58be3f) into branch-22.02 (967a333) will decrease coverage by 0.00%.
The diff coverage is 12.35%.

@@               Coverage Diff                @@
##           branch-22.02    #9796      +/-   ##
================================================
- Coverage         10.49%   10.48%   -0.01%     
================================================
  Files               119      119              
  Lines             20305    20344      +39     
================================================
+ Hits               2130     2133       +3     
- Misses            18175    18211      +36     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/column.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <ø> (ø)
python/cudf/cudf/core/indexed_frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/multiindex.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/utils.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/csv.py	96.00% <100.00%> (+0.10%)	⬆️
python/dask_cudf/dask_cudf/io/tests/test_csv.py	100.00% <100.00%> (ø)

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[rjzamora]

rerun tests

[rjzamora]

@gpucibot merge