clk: mediatek: make dpi0_sel propagate rate changes

This mux is supposed to select a fitting divider after the PLL
is already set to the correct rate.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: James Liao <jamesjj.liao@mediatek.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>