drm/gud: Use the shadow plane helper

Use the shadow plane helper to take care of mapping the framebuffer for
CPU access. The synchronous flushing is now done inline without the use of
a worker. The async path now uses a shadow buffer to hold framebuffer
changes and it doesn't read the framebuffer behind userspace's back
anymore.

v2:
- Use src as variable name for iosys_map (Thomas)
- Prepare imported buffer for CPU access in the driver (Thomas)

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221122-gud-shadow-plane-v2-5-435037990a83@tronnes.org