Rocket's QoR has improved enough that the FMAs are on the critical
path. This change seems to keep the integer pipeline's logic
paths balanced with the FPU.
The previous approach used ex_reg_valid to determine whether to
source data from the FPU or RoCC. Thus, when the RoCC was not
present, it was still creating muxes. Using ex_cp_valid instead
gets rid of them.
