drm/msm/dpu: ignore NULL clocks

This isn't an error.  Also the clk APIs handle the NULL case, so we can
just delete the check.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Tested-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>