Rounding the porches up instead of down fixes the samsung-dsim at
some more resolutions and refresh rates:

The following resolutions are working with rounded-up porches, but don't
work when the porches are rounded down:

1920x1080-59.94
1920x1080-30.00
1920x1080-29.97
1920x1080-25.00
1680x1050-59.88
1280x1024-75.02
 1200x960-59.99
 1280x720-50.00
 1024x768-75.03
 1024x768-60.00
  640x480-60.00
  640x480-59.94

Reviewed-by: Adam Ford <aford173@gmail.com>  #imx8mm-beacon
Tested-by: Adam Ford <aford173@gmail.com>  #imx8mm-beacon
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-v2-4-846603df0e0a@pengutronix.deSigned-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230818-samsung-dsim-v2-4-846603df0e0a@pengutronix.de