OMAP: DSS2: OMAPFB: make DBG() more resistant in if-else constructions

When DBG() is used in a simple if-else, the resulting code path
currently depends on the definition of DBG(). Inserting the statement in
a "do { ... } while (0)" prevents this possible misuse.

Signed-off-by: Niels de Vos <ndevos@redhat.com>
[tomi.valkeinen@ti.com: changed the title of the commit msg]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>