The field is assigned but never read, remove it.

This fixes a bug caused by the struct vb2_buffer field not being be the
very first field of the vsp1_video_buffer buffer structure as required
by videobuf2.

Cc: stable@vger.kernel.org
Reported-by: Takanari Hayama <taki@igel.co.jp>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>