tools/power x86_energy_perf_policy: Enhances SoC Slider related checks

tools/power x86_energy_perf_policy: Enhances SoC Slider related checks

When processor_thermal_soc_slider is loaded, its slider
and offset modparams are visible.  Check that the driver
actually registered the profile named "SoC Slider" before
reading or writing these modparams.

n.b. This utility allows writing the Slider and Offset modparams
even if the driver policy is not "balanced".  Currently the
processor_thermal_soc_slider consults those modparams
only in "balanced" mode.

Signed-off-by: Len Brown <len.brown@intel.com>