drm/gma500: Add fb gtt offset to fb base

Old code assumed framebuffer starts at base of stolen memory. Since the
addition of hardware cursors, this might not be true anymore so add the
gtt offset to the calculation.

Reported-by: Holger Schurig <holgerschurig@gmail.com>
Tested-by: Holger Schurig <holgerschurig@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>