Move the static keyword to the front of declaration of modes,
and resolve the following compiler warning that can be seen
when building with warnings enabled (W=1):

drivers/gpu/drm/exynos/exynos_mixer.c:1074:2: warning:
  ‘static’ is not at beginning of declaration [-Wold-style-declaration]
Signed-off-by: Krzysztof Wilczynski <kw@linux.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>