kgsl_pwrscale: Rework policy sysfs implementation

In the current implementation, the pwrscale policy kobj is
created when the pwrscale driver initializes.  It is then added
to the kobj hierarchy when a governor adds sysfs files.

When a governor removes sysfs files, it frees the kobj.

Then, when a new governor starts up, it attempts to add an
uninitialized kobj.  This fails.

Instead, use a constant kobj for the policy directory,
and add governor-specific attributes under it.  If a
sysfs group is given a name, all attributes in the group
will appear in a subdirectory with that name (see cpufreq
governor tuning variables).  Use this approach for our
governor tuning.

TODO (possibly in a future patch) - Rework this so the
governor attributes can be groups attached directly to
the pwrscale kobj.

Change-Id: I7b8ae6d5e690ee107be037ccaf2e737e52062feb