OMAPDSS: DISPC: fix sparse warning

Fix sparse warning:

drivers/video/omap2/dss/dispc.c:3320:6: warning: symbol
'dispc_dump_irqs' was not declared. Should it be static?

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>