fbdev: Use _IOMEM_ infix for I/O-memory helpers

Change the infix for fbdev's I/O-memory helpers from _IO_ to _IOMEM_
to distiguish them from other types of I/O, such as file operations.
The helpers operate on memory ranges in the I/O address space and the
naming should make this clear. Adapt all users. No functional changes.

Suggested-by: Helge Deller <deller@gmx.de>
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/20230729193157.15446-2-tzimmermann@suse.de