drm/tiny/st7586: Use fbdev-dma

Implement fbdev emulation with fbdev-dma. Fbdev-dma now supports
damage handling, which is required by st7586. Avoids the overhead of
fbdev-generic's additional shadow buffering. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Lechner <david@lechnology.com>
Acked-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-41-tzimmermann@suse.de