cpufreq: Avoid creating excessively large stack frames

In the process of modifying a cpufreq policy, the cpufreq core makes
a copy of it including all of the internals which is stored on the
CPU stack.  Because struct cpufreq_policy is relatively large, this
may cause the size of the stack frame to exceed the 2 KB limit and
so the GCC complains when -Wframe-larger-than= is used.

In fact, it is not necessary to copy the entire policy structure
in order to modify it, however.

First, because cpufreq_set_policy() obtains the min and max policy
limits from frequency QoS now, it is not necessary to pass the limits
to it from the callers.  The only things that need to be passed to it
from there are the new governor pointer or (if there is a built-in
governor in the driver) the "policy" value representing the governor
choice.  They both can be passed as individual arguments, though, so
make cpufreq_set_policy() take them this way and rework its callers
accordingly.  This avoids making copies of cpufreq policies...