drm/vram-helper: remove populate/unpopulate

The default path for populate/unpopulate is already this.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-8-airlied@gmail.com