drm/aspeed: Use GEM DMA fbdev emulation

Use the fbdev emulation that is optimized for DMA helpers. Avoids
possible shadow buffering and makes the code simpler.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230313155138.20584-5-tzimmermann@suse.de