The IPU that drives HDMI must have its pre_sel set to pll2_pfd_396m
to avoid stepping on the LVDS output's toes, as the PLL can't be clocked
to the pixel clock and to the LVDS serial clock (3.5*pixel clock) at the
same time.

As we are using ipu1_di0 and ipu2_di0, ensure both are switched to
to pll2_pfd2_396m to avoid issues. The LDB driver will switch the
required IPU to ldb_di1 when it uses it to drive LVDS.
Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Signed-off-by: Romain Perier <romain.perier@collabora.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>