drm/exynos: Make exynos4/5_fimd_driver_data static

Fixes the following sparse warnings:
drivers/gpu/drm/exynos/exynos_drm_fimd.c:65:25: warning:
symbol 'exynos4_fimd_driver_data' was not declared. Should it be static?
drivers/gpu/drm/exynos/exynos_drm_fimd.c:69:25: warning:
symbol 'exynos5_fimd_driver_data' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>