drm: drm_atomic_crtc_get_property should be static

Fixes the following sparse warning:
 drivers/gpu/drm/drm_atomic.c:442:5: warning: symbol
 'drm_atomic_crtc_get_property' was not declared. Should it be static?

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_atomic.c

@@ -438,7 +438,8 @@ EXPORT_SYMBOL(drm_atomic_crtc_set_property);
  * consistent behavior you must call this function rather than the
  * driver hook directly.
  */
-int drm_atomic_crtc_get_property(struct drm_crtc *crtc,
+static int
+drm_atomic_crtc_get_property(struct drm_crtc *crtc,
 		const struct drm_crtc_state *state,
 		struct drm_property *property, uint64_t *val)
 {