[DataGridPremium] Fix autosize grouping cell

[romgrk]

Closes #10848

Fix the CSS width property so that the width calculation logic can figure it out properly.

Before: https://codesandbox.io/p/sandbox/async-sea-4r2smt After: https://codesandbox.io/p/sandbox/strange-browser-d7x9ml

[mui-bot]

Deploy preview: https://deploy-preview-11870--material-ui-x.netlify.app/

Generated by :no_entry_sign: dangerJS against 7c5229c279ee1078ef77077b70c743b564c2df93

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This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions[bot]]

This pull request has conflicts, please resolve those before we can evaluate the pull request.