MAINTAINERS: Broaden scope of simpledrm entry

There will be more DRM drivers for firmware-provided framebuffers. Use
the existing entry for simpledrm instead of adding a new one for each
driver. Also add DRM's aperture helpers, which are part of the driver's
infrastructure.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220622140134.12763-2-tzimmermann@suse.de