[media] drx-j: Properly initialize mpeg struct before using it

The cfg_mpeg_output has more fields than what it is initialized
when the code is called. Be sure to initialize everything before
use, in order to avoid random behaviors.

Acked-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>