drm/nouveau/tegra: remove needless NULL check

There has already been NULL check in clk_prepare_enable() and
clk_disable_unprepare(), so remove needless NULL check before
calling them.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220519072950.128268-1-guozhengkui@vivo.com