drm/exynos: Make g2d_userptr_get_dma_addr static

Fixes the following warning:
drivers/gpu/drm/exynos/exynos_drm_g2d.c:327:12: warning:
symbol 'g2d_userptr_get_dma_addr' 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>