fbdev: Document that framebuffer_alloc() returns zero'ed data

Most fbdev drivers depend on framebuffer_alloc() to initialize the
allocated memory to 0. Document this guarantee.

v3:
* slightly reword the sentence (Miguel)

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Cc: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230715185343.7193-19-tzimmermann@suse.de

fbdev/core: Move framebuffer and backlight helpers into separate files

Move framebuffer and backlight helpers into separate files. Leave
fbsysfs.c to sysfs-related code. No functional changes.

The framebuffer helpers are not in fbmem.c because they are under
GPL-2.0-or-later copyright, while fbmem.c is GPL-2.0.

v2:
* include <linux/mutex.h> (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613110953.24176-34-tzimmermann@suse.de