drm/stm: dsi: provide the implementation of mode_valid()

The dsi has several constraints on the video modes it can support,
mainly due to the frequencies that can be generated by the PLL
integrated in the DSI device.

Verify that the required HS clock can be generated by the PLL.

The dsi clock from the dsi PLL and the ltdc pixel clock are
asynchronous. The dsi needs to return in LP mode during HFP or HBP
to re-synchronize at each video line.

Verify that the duration of HFP and HBP allows the dsi to enter in
LP mode.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Acked-by: Philippe Cornu <philippe.cornu@foss.st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@foss.st.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211218215055.212421-3-antonio.borneo@foss.st.com