plugins/amdgpu: Cleanup env variable handling

[tursulin]

This is not saving a lot of lines, just two, but perhaps the pattern of:

  kfd_fw_version_check = getenv_bool("KFD_FW_VER_CHECK", true);

Is somewhat cleaner than:

  /* Default Values */
  kfd_fw_version_check = true;
...
  getenv_bool("KFD_FW_VER_CHECK", &kfd_fw_version_check); 

I also make the helpers static and demote parsing errors to warnings since the execution does continue.

I am locally toying with the idea of adding a new env variable to ignore gpu id hence I spotted this cleanup opportunity.

cc @fdavid-amd

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.22%. Comparing base (9a1453b) to head (9788da3).
⚠️ Report is 1 commits behind head on criu-dev.

Additional details and impacted files

@@             Coverage Diff              @@
##           criu-dev    #3014      +/-   ##
============================================
- Coverage     57.22%   57.22%   -0.01%     
============================================
  Files           154      154              
  Lines         40440    40441       +1     
  Branches       8863     8863              
============================================
  Hits          23142    23142              
- Misses        17034    17035       +1     
  Partials        264      264              

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[avagin]

LGTM