drm/gma500: Remove unused declarations

These declarations are not implemented now, remove them.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230817133754.36524-1-yuehaibing@huawei.com