drm/bridge: samsung-dsim: reread ref clock before configuring PLL

The PLL reference clock may change at runtime when its parent clock
changes. For example, this may happen on the i.MX8M Nano if the
reference clock is a child of the Video PLL. If the pixel clock changes,
this may propagate to the Video PLL and as a side effect change the
reference clock. Thus, reading the clock rate during probe is not
sufficient to correctly configure the PLL for the expected hs clock.

Read the actual rate of the reference clock before calculating the PLL
configuration parameters.

Note that the "samsung,pll-clock-frequency" is always preferred and PLL
reference clock is only read from the clock tree if that device tree
property is not set.

Reviewed-by: Inki Dae <inki.dae@samsung.com>
Acked-by: Inki Dae <inki.dae@samsung.com>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de> # Kontron BL i.MX8MM + Waveshare 10.1inch HDMI LCD (E)
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20230818-samsung-dsim-v2-2-846603df0e0a@pengutronix.de


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230818-samsung-dsim-v2-2-846603df0e0a@pengutronix.de