drm/exynos: fix g2d_open/close helper function definitions

drm/exynos: fix g2d_open/close helper function definitions

The empty stub functions are defined as global functions, which
causes a warning because of missing prototypes:

drivers/gpu/drm/exynos/exynos_drm_g2d.h:37:5: error: no previous prototype for 'g2d_open'
drivers/gpu/drm/exynos/exynos_drm_g2d.h:42:5: error: no previous prototype for 'g2d_close'

Mark them as 'static inline' to avoid the warning and to make
them behave as intended.

Fixes: eb4d9796fa34 ("drm/exynos: g2d: Convert to driver component API")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>