[media] v4l2: replace video op g_mbus_fmt by pad op get_fmt

The g_mbus_fmt video op is a duplicate of the pad op. Replace all uses
by the get_fmt pad op and remove the video op.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Prabhakar Lad <prabhakar.csengg@gmail.com>
Cc: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>