ACPICA code takes care about cache flushing on S1/S2/S3 in
acpi_hw_extended_sleep() and acpi_hw_legacy_sleep().

acpi_suspend_enter() calls into ACPICA code via acpi_enter_sleep_state()
for S1 or x86_acpi_suspend_lowlevel() for S3.

acpi_sleep_prepare() call tree:
  __acpi_pm_prepare()
    acpi_pm_prepare()
      acpi_suspend_ops::prepare_late()
      acpi_hibernation_ops::pre_snapshot()
      acpi_hibernation_ops::prepare()
    acpi_suspend_begin_old()
      acpi_suspend_begin_old::begin()
  acpi_hibernation_begin_old()
    acpi_hibernation_ops_old::acpi_hibernation_begin_old()
  acpi_power_off_prepare()
    pm_power_off_prepare()

Hibernation (S4) and Power Off (S5) don't require cache flushing, so
the only interesting callsites are acpi_suspend_ops::prepare_late()
and acpi_suspend_begin_old::begin(). Both of them have cache flush
on ->enter() operation in acpi_suspend_enter().

Remove redundant ACPI_FLUSH_CPU_CACHE() in acpi_sleep_prepare() and
acpi_suspend_enter().
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>