drm/tegra: Use fbdev DMA helpers

Use fbdev's DMA helpers for fbdev emulation. 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>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Mikko Perttunen <mperttunen@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230707083422.18691-4-tzimmermann@suse.de