drm/exynos: staticize exynos_drm_gem_init()

The exynos_drm_gem_init() is used only in exynos_drm_gem.c file. Make it
static and don't export it.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>