Avoid dfs transformer checking in feat importance fast mode check if pipline is stacked ensemble

[tamargrey]

fixes #4082

[tamargrey]

The other option for fixing was to change self.component_graph[DFSTransformer.name] to use whatever name is actually being used in the component graph for the dfs transformer(s), but I don't know of any other way the name could be anything other than "DFS Transformer" other than the stacked ensemble case, so this seemed simpler.

[codecov]

Codecov Report

Merging #4083 (1ec4228) into main (345c05c) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #4083   +/-   ##
=====================================
  Coverage   99.7%   99.7%           
=====================================
  Files        349     349           
  Lines      37548   37548           
=====================================
  Hits       37429   37429           
  Misses       119     119           

Impacted Files	Coverage Δ
...understanding_tests/test_permutation_importance.py	100.0% <ø> (ø)
evalml/pipelines/pipeline_base.py	98.5% <100.0%> (ø)

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