[dask] Fix prediction on DaskDMatrix with multiple meta data.

[trivialfis]

• Unify the meta info handling for training and prediction.

[trivialfis]

cc @kylejn27

[codecov-io]

Codecov Report

Merging #6333 into master will decrease coverage by 0.01%.
The diff coverage is 97.87%.

@@            Coverage Diff             @@
##           master    #6333      +/-   ##
==========================================
- Coverage   80.84%   80.82%   -0.02%     
==========================================
  Files          12       12              
  Lines        3373     3375       +2     
==========================================
+ Hits         2727     2728       +1     
- Misses        646      647       +1     

Impacted Files	Coverage Δ
python-package/xgboost/dask.py	81.14% <97.87%> (-0.12%)	⬇️

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