thermal: gov_power_allocator: Rearrange initialization of local variables

Rearrange the initialization of local variables in allocate_power() so
as to improve code clarity and the visibility of the initial values.

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>