drm/i915: fixup sbi_read/write locking

commit 09153000
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Dec 12 14:06:44 2012 +0100

    drm/i915: rework locking for intel_dpio|sbi_read|write

reworked the locking around sbi_read/write functions for 3.8-fixes.
But

commit dde86e2d
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Sat Dec 1 12:04:25 2012 -0200

    drm/i915: add lpt_init_pch_refcl

Added new use-cases in the -next tree which has not been updated in
the merge. Fix it up.

Reported-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Tested-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>