fix passing axes to plot_density with several datasets

[aloctavodia]

Fixes #1197

• Follows official PR format
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1198 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1198      +/-   ##
==========================================
+ Coverage   93.11%   93.12%   +0.01%     
==========================================
  Files          94       94              
  Lines        9478     9478              
==========================================
+ Hits         8825     8826       +1     
+ Misses        653      652       -1     

Impacted Files	Coverage Δ
arviz/plots/backends/matplotlib/densityplot.py	98.21% <100.00%> (+1.78%)	⬆️

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