drivers/gpu/drm/i915/intel_lvds.c: fix locking snafu

s/unlock/lock/

Addresses http://bugzilla.kernel.org/show_bug.cgi?id=12575

Reported-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Dave Airlie <airlied@linux.ie>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>