drm/mcde: Use GEM DMA fbdev emulation

drm/mcde: Use GEM DMA fbdev emulation

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

Reported-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/dri-devel/CACRpkdawSQsNqKJkSSoSw3HmMHyNXFUywxkdszpTC-a_uZA+tQ@mail.gmail.com/
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230313155138.20584-25-tzimmermann@suse.de