minnowmax: Enable the copy framebuffer

Update the video driver to support this feature and enable it on
minnowmax to speed up the display.

With this change, the time taken to print the environment to the display
without CONFIG_CONSOLE_SCROLL_LINES is reduced from over 13 seconds to
300ms, at 1280x1024.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>