karo: tx6: set LCD x, y dimensions to 0 when disabling LCD

Even when the LCD is disabled, the LCD framework will still clear the
video memory. This can take a cosiderable amount of time for the
maximum resolution of 16MiB that the i.MX6 supports.

Setting vl_row and vl_col to 0 results in a zero sized framebuffer
to avoid this unnecessary delay.