drm/i915: Contain the WA_REG macro

Prevent leaking the if scoping by containing the WA_REG
macro inside its own scope.

Reported-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
[danvet: Appease checkpatch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>