drm/gma500: Remove GTT roll support

GTT roll support was used to accelerate fb panning on some machines.
Unfortunately this never worked properly with multiple monitors and
caused issues on others where the framebuffer wouldn't fit in stolen
memory. Let's remove it!

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028143608.1284-1-patrik.r.jakobsson@gmail.com