drm/gma500: Remove fbdev vma open and close callbacks

Remove fbdev emulations' open and close implementaitons for the VM. The
functions are empty. Also update the naming of the VMA code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230313151610.14367-4-tzimmermann@suse.de