drm/exynos: fix NULL pointer reference

There is a case called disable_plane callback function even if
plane->crtc is NULL from exynos_drm_encoder_disable and it will cause
NULL pointer reference error.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>