sched/psi: Optimise psi_group_change a bit

The current code loops over the psi_states only to call a helper which
then resolves back to the action needed for each state using a switch
statement. That is effectively creating a double indirection of a kind
which, given how all the states need to be explicitly listed and handled
anyway, we can simply remove. Both the for loop and the switch statement
that is.

The benefit is both in the code size and CPU time spent in this function.
YMMV but on my Steam Deck, while in a game, the patch makes the CPU usage
go from ~2.4% down to ~1.2%. Text size at the same time went from 0x323 to
0x2c1.

Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lkml.kernel.org/r/20240625135000.38652-1-tursulin@igalia.com