Fix freq warn and igt/tests/xe/xe_guc_pc freq tests on DG2.

While trying to add D3Cold support using a DG2, I noticed some bizarre warnings related to the freq. It ends up that on this particular DG2 in RIL: https://gtax-ril-fm.intel.com/#/clients/2189 we have PCODE deciding for an efficient freq that is lower than the fused minimal one. Since PCODE is the ultimate decider of the frequency, let's not fight and accept it, by adjusting our minimal rpn everytime that we see a lower rpe. Also, since this is prune to change, let's update it before reading back.