[media] v4l: s5p-tv: mixer: fix handling of interlaced modes

The next frame was fetched by Mixer at every VSYNC event.  This caused tearing
when Mixer's output in interlaced mode.  This patch fixes this bug by fetching
new frame every second VSYNC when working in interlaced mode.

Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>