drm/stm: Remove usage of drm_display_mode_to_videomode()

There is not much value in the extra conversion step, the calculations
required for the LTDC IP are different than what is used in the
drm_display_mode_to_videomode(), so just do the right ones in the LTDC
driver right away.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Antonio Borneo <antonio.borneo@foss.st.com>
Cc: Benjamin Gaignard <benjamin.gaignard@foss.st.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Philippe Cornu <philippe.cornu@foss.st.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Vincent Abriou <vincent.abriou@foss.st.com>
Cc: Yannick Fertre <yannick.fertre@foss.st.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-stm32@st-md-mailman.stormreply.com
To: dri-devel@lists.freedesktop.org
Tested-by: Yannick Fertré <yannick.fertre@st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210607172457.14471-1-marex@denx.de