drm/exynos: Staticize local function in exynos_drm_gem.c

The exynos_drm_gem_mmap_buffer() is not used outside so make it static.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>