fbdev: Move default fb_mmap code into helper function

Move the default fb_mmap code for I/O address spaces into the helper
function fb_io_mmap(). The helper can either be called via struct
fb_ops.fb_mmap or as the default if no fb_mmap has been set. Also
set the new helper in __FB_DEFAULT_IOMEM_OPS_MMAP.

In the mid-term, fb_io_mmap() is supposed to become optional. Fbdev
drivers will initialize their struct fb_ops.fb_mmap to the helper
and select a corresponding Kconfig token. The helper can then be made
optional at compile time.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127131655.4020-31-tzimmermann@suse.de