thermal: gov_power_allocator: Use shorter paths to access data when possible

The 'cdev' pointer in allow_maximum_power() is valid, so there is no
need to use 'instance->cdev' instead of it.

This change is not expected to alter the general functionality.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>