We'll need this when we merge PC8 and Runtime PM: the PC8
enable/disable functions need that lock.

Also, it's good practice to not hold a lock for longer than strictly
needed.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>