drm/gma500: remove stub .open/postclose

These are unused and can safely be dropped. DRM core verifies they're
non-NULL before it calls them.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>