drm/gud: Prepare buffer for CPU access in gud_flush_work()

In preparation for moving to the shadow plane helper prepare the
framebuffer for CPU access as early as possible.

v2:
- Use src as variable name for iosys_map (Thomas)

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
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-v2-4-435037990a83@tronnes.org