fb.h: Provide alternate screen_base pointer

Some drivers use member screen_base of struct fb_info to store non-
__iomem pointers, creating the need for ugly __force typecasts to
avoid sparse warnings. This adds an alternate pointer without the
__iomem qualifyer for this use.

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