video: mxsfb: introduce lcdif_power_down

Introudce a new function lcdif_power_down.

1. Waits for a VSYNC interrupt to guarantee the reset is done at the
   VSYNC edge, which somehow makes the LCDIF consume the display FIFO(?)
   and helps the LCDIF work normally at the kernel stage.
2. Add power down function to stop lcdif.

The reason to introduce lcdif_power_down is that we want lcdif to be in
initial state when doing uboot reset or before kernel boot to make
system stable, otherwise system may hang.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
Acked-by: Anatolij Gustschin <agust@denx.de>