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 <[e-mail]>
Reviewed-by: Andi Shyti <[e-mail]>
Signed-off-by: Inki Dae <[e-mail]>

drivers/gpu/drm/exynos/exynos_drm_g2d.h

···
 	return -ENODEV;
 }
 
-int g2d_open(struct drm_device *drm_dev, struct drm_file *file)
+static inline int g2d_open(struct drm_device *drm_dev, struct drm_file *file)
 {
 	return 0;
 }
 
-void g2d_close(struct drm_device *drm_dev, struct drm_file *file)
+static inline void g2d_close(struct drm_device *drm_dev, struct drm_file *file)
 { }
 #endif