staging: fbtft: access screen buffer directly

In fbtft-bus.c:fbtft_write_vmem16_bus9(), ioread8() is used for
accessing the provided screen array. Since screen_buffer actually
points to an ordinary buffer, instead access it directly.

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