OMAPDSS: DISPC: Fix the usage of wait_for_completion_timeout

The return value of wait_for_completion_timeout() is always
>= 0 with unsigned int type.

So the condition "ret < 0" or "ret >= 0" is pointless.

Signed-off-by: liu chuansheng <chuansheng.liu@intel.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>