staging/fbtft: Use fb_ops helpers for deferred I/O

Generate callback functions for struct fb_ops with the fbdev macro
FB_GEN_DEFAULT_DEFERRED_SYSMEM_OPS(). Initialize struct fb_ops to
the generated functions with an fbdev initializer macro.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230828132131.29295-9-tzimmermann@suse.de