OMAP: DSS2: OMAPFB: Add some locking debug checks

Trigger WARN_ON() messages from various places in the code in case the
memory region is not currently locked.

Signed-off-by: Ville Syrjälä <ville.syrjala@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>