[media] exynos-gsc: Register v4l2 device

Gscaler video device registration was happening without reference to
a parent v4l2_dev causing probe to fail. The patch creates a parent
v4l2 device and uses it for the gsc m2m video device registration.
This fixes regression introduced with comit commit 1c1d86a1
[media] v4l2: always require v4l2_dev, rename parent to dev_parent

Signed-off-by: Arun Kumar K <arun.kk@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Cc: stable@vger.kernel.org