We only need to check for this in psr_enable, everything else is
already protect by the dev_priv->psr.enabled checks. Those need the
psr locking, but these functions are called infrequent enough that the
locking overhead is negligible.

Suggested by Chris Wilson.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>