media: vsp1/vsp1_drm.h: add missing 'struct' kernel-doc keyword

The kernel-doc documentation for two structs was missing the
'struct' keyword.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>