drm/i915: Remove unneeded struct_mutex around rpm

We don't need struct_mutex for acquiring an rpm wakeref, and do not need
to serialise those register read (it's the wrong mutex for those
registers in any case). Begone!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170218150050.10414-1-chris@chris-wilson.co.uk


Reviewed-by: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>