drm/meson: gate px_clk when setting rate

Disable the px_clk when setting the rate to recover a fully
configured and correctly reset VCLK clock tree after the rate
is set.

Fixes: 77d9e1e6

 ("drm/meson: add support for MIPI-DSI transceiver")
Reviewed-by: Nicolas Belin <nbelin@baylibre.com>
Link: https://lore.kernel.org/r/20240403-amlogic-v6-4-upstream-dsi-ccf-vim3-v12-4-99ecdfdc87fc@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240403-amlogic-v6-4-upstream-dsi-ccf-vim3-v12-4-99ecdfdc87fc@linaro.org