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>