iwlwifi: mvm: refactor power code

The main complexity of the power code is that it needs to
take into account the firmware limitations.
These limitations state that we need to have a global
picture of the vifs present in the system to be able to
decide if we can enable power management on a specific vif.

Even device power save (as opposed to vif power management)
must be disabled in certain circumstances (monitor vif).

Refactor the current code to make this clearer by defining
a function that explicitely computes these constraints.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>