fbdev: Remove default file-I/O implementations

Drop the default implementations for file read, write and mmap
operations. Each fbdev driver must now provide an implementation
and select any necessary helpers. If no implementation has been
set, fbdev returns an errno code to user space. The code is the
same as if the operation had not been set in the file_operations
struct.

This change makes the fbdev helpers for I/O memory optional. Most
systems only use system-memory framebuffers via DRM's fbdev emulation.

v2:
	* warn once if I/O callbacks are missing (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127131655.4020-33-tzimmermann@suse.de