video: mxsfb: call remove() when booting OS

Add DM_FLAG_OS_PREPARE flag to ensure that the driver's
remove() callback is invoked before booting the kernel.
This is required to stop the LCDIF controller. This was
the behaviour with old driver without DM_VIDEO support.
Without stopping the LCDIF we sometimes observe incorrect
Linux logo position.

Fixes: ae076058 ("imx: mx6ul_14x14_evk: convert to DM_VIDEO")
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reported-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>