drm/mediatek: Check return value of mtk_drm_ddp_comp_for_plane.

The mtk_drm_ddp_comp_for_plane can return NULL, but the usage doesn't
check for it. Add check for it.

Fixes: d6b53f68

 ("drm/mediatek: Add helper to get component for a plane")
Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>