fix(module:date-picker): improve the horizontal position adaptability

[wilsoncook]

close #1450

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #1450

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #1495 into master will increase coverage by <.01%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #1495      +/-   ##
==========================================
+ Coverage   96.92%   96.92%   +<.01%     
==========================================
  Files         222      222              
  Lines        9500     9506       +6     
  Branches     1296     1297       +1     
==========================================
+ Hits         9208     9214       +6     
  Misses         42       42              
  Partials      250      250

Impacted Files	Coverage Δ
...omponents/date-picker/abstract-picker.component.ts	98.52% <100%> (ø)	⬆️
components/date-picker/month-picker.component.ts	100% <100%> (ø)	⬆️
components/date-picker/picker.component.ts	97.53% <83.33%> (+0.12%)	⬆️
components/modal/nz-modal.component.ts	96.37% <0%> (+0.03%)	⬆️

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