drm/tiny/gm12u320: Use fbdev-shmem

Implement fbdev emulation with fbdev-shmem. Avoids the overhead of
fbdev-generic's additional shadow buffering. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-16-tzimmermann@suse.de