drm/tilcdc: Fix non static symbol warning

Fixes the following sparse warning:

drivers/gpu/drm/tilcdc/tilcdc_drv.c:64:5: warning:
 symbol 'tilcdc_atomic_check' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>