Add an fbdev emulation for SHMEM-based memory managers. The code is
similar to fbdev-generic, but does not require an additional shadow
buffer for mmap(). Fbdev-shmem operates directly on the buffer object's
SHMEM pages. Fbdev's deferred-I/O mechanism updates the hardware state
on write operations.

The memory pages of GEM SHMEM cannot be detected by fbdefio. Therefore
fbdev-shmem implements the .get_page() hook in struct fb_deferred_io.
The fbdefio helpers call this hook to retrieve the page directly from
fbdev-shmem instead of trying to detect it internally.

v3:
- clarify on get_page mechanism in commit description (Javier)
v2:
- use drm_driver_legacy_fb_format() (Geert)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-9-tzimmermann@suse.de