drm/msm/dp: set stream_pixel rate directly

The only clock for which we set the rate is the "stream_pixel". Rather
than storing the rate and then setting it by looping over all the
clocks, set the clock rate directly.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/474714/
Link: https://lore.kernel.org/r/20220217055529.499829-4-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>