video: speedup writing strings to fb console

With enabled framebuffer console the printenv command can take several
seconds to complete. This patch temporarily disables cache flush when
writing strings to fb console. Then there is no noticable delay anymore.

Tested with imx6 hdmi framebuffer.

Signed-off-by: Soeren Moch <smoch@web.de>
Acked-by: Stefano Babic <sbabic@denx.de>