PM: domains: Add curly braces to delimit comment + statement block

There is not strict need to group a comment and a single statement in an
if block, as comments are stripped by the pre-processor.  However,
adding curly braces does make the code easier to read, and may avoid
mistakes when changing the code later.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>