video/mx3fb: fix clock divider

Fix clock divider for COM57H5M10XRC display.
The previous setting caused flicker.

Tested on Qong (EVBLite with COM57H5M10XRC).

Signed-off-by: Stefano Babic <sbabic@denx.de>
Acked-by: Wolfgang Denk <wd@denx.de>
Acked-by: Anatolij Gustschin <agust@denx.de>