Make valid_state() check if the ->enter callback is present in
suspend_ops (only PM_SUSPEND_TO_IDLE can be valid otherwise) and
make sleep_state_supported() call valid_state() consistently to
validate the states other than PM_SUSPEND_TO_IDLE.

While at it, clean up the comment in valid_state().

No expected functional impact.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>