intel_pstate: Remove extra conversions in pid calculation

pid->setpoint and pid->deadband can be initialized in fixed point, so we
can avoid the int_tofp in pid_calc.

Signed-off-by: Philippe Longepe <philippe.longepe@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>