drm/aperture: Fix some kerneldoc comments

Reword some kerneldoc comments for the DRM aperture handling code.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220905162241.391226-1-thierry.reding@gmail.com