thermal: gov_bang_bang: Split bang_bang_control()

Move the setting of the thermal instance target state from
bang_bang_control() into a separate function that will be also called
in a different place going forward.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Peter Kästle <peter@piie.net>
Reviewed-by: Zhang Rui <rui.zhang@intel.com>
Cc: 6.10+ <stable@vger.kernel.org> # 6.10+
Link: https://patch.msgid.link/3313587.aeNJFYEL58@rjwysocki.net