Revert "PM: sleep: Do not assume that "mem" is always present"

Revert commit bfcc1e67 ("PM: sleep: Do not assume that "mem" is
always present"), because it breaks compatibility with user space
utilities assuming that "mem" will always be present in
/sys/power/state.

Fixes: bfcc1e67

 ("PM: sleep: Do not assume that "mem" is always present")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>