drm/todo: Clarify situation around fbdev and defio

The TODO item is misleading and makes it seem as if fbdev emulation
cannot be used with SHMEM. Rephrase the text to describe the current
situation more correctly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025092759.13069-3-tzimmermann@suse.de

Documentation/gpu/todo.rst

@@ -206,10 +206,10 @@ Generic fbdev defio support
 ---------------------------
 
 The defio support code in the fbdev core has some very specific requirements,
-which means drivers need to have a special framebuffer for fbdev. Which prevents
+which means drivers need to have a special framebuffer for fbdev. The main
-us from using the generic fbdev emulation code everywhere. The main issue is
+issue is that it uses some fields in struct page itself, which breaks shmem
-that it uses some fields in struct page itself, which breaks shmem gem objects
+gem objects (and other things). To support defio, affected drivers require
-(and other things).
+the use of a shadow buffer, which may add CPU and memory overhead.
 
 Possible solution would be to write our own defio mmap code in the drm fbdev
 emulation. It would need to fully wrap the existing mmap ops, forwarding