Revert "PM / Domains: Handle safely genpd_syscore_switch() call on non-genpd device"

Revert commit 8b55e55e (PM / Domains: Handle safely
genpd_syscore_switch() call on non-genpd device) which was misguided
(the change made by it was not necessary) and it introduced a call to
a function that may sleep into an atomic context code path.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>