drm/gma500: Do not clear framebuffer GEM objects during cleanup

Gma500 unnecessarily clears the framebuffer's GEM-object pointer
before calling drm_framebuffer_cleanup(). Remove this code to make
gma500 consistent with the rest of the drivers.

The change is cosmetic, as drm_framebuffer_cleanup() does not
touch the object pointer on gma500.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://lore.kernel.org/r/20250904121157.395128-1-tzimmermann@suse.de