[media] s5p-mfc: remove unnecessary calling to function video_devdata()

Since we have get vdev by calling video_devdata() at the beginning of
s5p_mfc_open(), we should just use vdev instead of calling video_devdata()
again in the following code.

Signed-off-by: Zhaowei Yuan <zhaowei.yuan@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>