Made g_tune_pme work correctly when ir->init_step > 0

Made g_tune_pme work correctly when ir->init_step > 0

For .tpr files with ir->init_step > 0, the benchmarks do not have the
correct number of steps. This fix sets init_step to zero for the benchmark .tpr
files but keeps the original init_step value for the launch .tpr file.
Benchmarking should now work correctly for .tpr files any init_step value,
regardless whether the .cpt file was provided to g_tune_pme or not.

Change-Id: I9d44647d4c71e1514709ee4d6477bd416f455f42