staging: fbtft: use alternate screen pointer

Member screen_base in struct fb_info is declared with __iomem
qualifier causing sparse warnings when used as a regular ponter.
To avoid the warnings, instead use alternate non-__iomem pointer,
screen_buffer, troughout the driver.

Signed-off-by: Lars Svensson <lars1.svensson@sonymobile.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>