sm501fb: Ensure panel interface is not tristated when setup

When we setup the panel interface whilst configuring the
framebuffer, we should ensure the panel interface is not
in tristate, in case the bootloader or previous setup has
not enabled it.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>