Use the existing generator macros to create deferred-I/O helpers
for xen-fbfront and set them in the fb_ops structure. Functions
for damage handling on memory ranges and areas are provided by
the driver.

Xen-fbfront's implementation of fb_write writes to system memory,
so the generated code can use the respective helper internally.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230706151432.20674-11-tzimmermann@suse.de