drm/exynos: Make g2d_pm_ops static

Fixes the following warning:
drivers/gpu/drm/exynos/exynos_drm_g2d.c:897:1: warning:
symbol 'g2d_pm_ops' 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>