Revert "ACPI: processor: idle: Only flush cache on entering C3"

Revert commit 87ebbb8c ("ACPI: processor: idle: Only flush cache
on entering C3") that broke the assumptions of the acpi_idle_play_dead()
callers.

Namely, the CPU cache must always be flushed in acpi_idle_play_dead(),
regardless of the target C-state that is going to be requested, because
this is likely to be part of a CPU offline procedure or preparation for
entering a system-wide sleep state and the lack of synchronization
between the CPU cache and RAM may lead to problems going forward, for
example when the CPU is brought back online.

In particular, it breaks resume from suspend-to-RAM on Lenovo ThinkPad
C13 which fails occasionally until the problematic commit is reverted.

Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
[ rjw: Changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>