pm: device_runtime: Use pm flags for runtime state

Although we are declaring `pm->enable`as bitfield, it ends up using
more memory due memory alignment.

Since we already have an atomic variable for device flags, this commit
adds a new flag to indicates whether or not device runtime is enabled.
Doing it we are saving some extra bits and avoiding need to lock the
mutex in several situations since we can atomically check if pm
runtime is enabled on a given device.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>