prevent internal call to has_prepared_data showing a warning

[awaelchli]

What does this PR do?

On master, we see the following warning when using a datamodule:

/Users/adrian/repositories/pytorch-lightning/pytorch_lightning/core/datamodule.py:167: LightningDeprecationWarning: DataModule property `has_prepared_data` was deprecated in v1.4 and will be removed in v1.6.

I believe the problem was introduced here #7657, but the warning wasn't shown for a while, cc @carmocca

Note: only in master, 1.3.x not affected. No changelog required.

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[codecov]

Codecov Report

Merging #8271 (8c35b9c) into master (8193bae) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8271   +/-   ##
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- Coverage      93%     92%   -0%     
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  Files         212     212           
  Lines       13720   13720           
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- Hits        12751   12684   -67     
- Misses        969    1036   +67