[media] omap3isp: Move non-critical code out of the mutex-protected section

The isp_video_pix_to_mbus() and isp_video_mbus_to_pix() calls in
isp_video_set_format() only access static fields of the isp_video
structure. They don't need to be protected by a mutex.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Enrico Butera <ebutera@users.sourceforge.net>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>