drm/bridge: ti-sn65dsi86: remove redundant null check

Because clk_disable_unprepare already checked NULL clock parameter,
so the additional checks are unnecessary, just remove it

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200930081859.52431-1-vulab@iscas.ac.cn