drm/exynos: Move static keyword to the front of declaration

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>