ACPI: simplify deferred execution path

We had two functions, acpi_os_execute_deferred() and
acpi_os_execute_hp_deferred() that differed only in that the
latter did acpi_os_wait_events_complete(NULL) before executing
the deferred function.

This patch consolidates those two functions and uses a flag in
the struct acpi_os_dpc to determine whether to do the wait.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>