gpu_enabled needs to be true only when gpu is used

[jedwards4b]

Only set gpu_enabled if gpus are actually used.

Test suite: PFS_Ld5.ne30pg3_ne30pg3_mg17.FLTHIST_v0d.derecho_intel
Test baseline:
Test namelist changes:
Test status: bit for bit
Fixes #4476

User interface changes?:

Update gh-pages html (Y/N)?:

[sjsprecious]

Thanks @jedwards4b for finding this issue. Previously I can assign the CPU job to the CPU node correctly by always setting the GPU_TYPE variable at https://github.com/ESMCI/cime/blob/master/CIME/case/case.py#L1588 (i.e., remove the if check). But I tried your fix and it works as well. I do not know which way is better and I will leave it to you to make the decision. Sorry that I did not catch this error earlier.