drm/fbdev-dma: Use fbdev DMA helpers

Use fbdev's DMA helpers for fbdev-dma. They are equivalent to the
previously used system-memory helpers, so no functional changes here.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230707083422.18691-3-tzimmermann@suse.de