drm/gma500: Remove unused function psb_gem_get_aperture()

Apparently, the function was never used at all.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201106124224.21201-1-tzimmermann@suse.de