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>

drivers/gpu/drm/i915/intel_display.c

@@ -4873,6 +4873,8 @@ static void lpt_init_pch_refclk(struct drm_device *dev)
 	if (!has_vga)
 		return;
 
+	mutex_lock(&dev_priv->dpio_lock);
+
 	/* XXX: Rip out SDV support once Haswell ships for real. */
 	if (IS_HASWELL(dev) && (dev->pci_device & 0xFF00) == 0x0C00)
 		is_sdv = true;
@@ -5015,6 +5017,8 @@ static void lpt_init_pch_refclk(struct drm_device *dev)
 	tmp = intel_sbi_read(dev_priv, SBI_DBUFF0, SBI_ICLK);
 	tmp |= SBI_DBUFF0_ENABLE;
 	intel_sbi_write(dev_priv, SBI_DBUFF0, tmp, SBI_ICLK);
+
+	mutex_unlock(&dev_priv->dpio_lock);
 }
 
 /*