drm/vram-helpers: Merge code into a single file

Most of the documentation was in an otherwise empty file, which was
probably just left from a previous clean-up effort. So move code and
documentation into a single file.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200331081238.24749-1-tzimmermann@suse.de