drm/panel/panel-sitronix-st7701: Fix RTNI calculation

The RTNI field is multiplied by 16 and incremented by 512 before being
used as the minimum number of pixel clock per horizontal line, hence
it is necessary to subtract those 512 bytes from htotal and then divide
the result by 16 before writing the value into the RTNI field. Fix the
calculation.

Fixes: de2b4917 ("drm/panel/panel-sitronix-st7701: Infer horizontal pixel count from TFT mode")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221012221159.88397-1-marex@denx.de