drivers/firmware: Move sysfb_init() from device_initcall to subsys_initcall_sync

Consider a configuration like this:
1, efifb (or simpledrm) is built-in;
2, a native display driver (such as radeon) is also built-in.

As Javier said, this is not a common configuration (the native display
driver is usually built as a module), but it can happen and cause some
trouble.

In this case, since efifb, radeon and sysfb are all in device_initcall()
level, the order in practise is like this:

efifb registered at first, but no "efi-framebuffer" device yet. radeon
registered later, and /dev/fb0 created. sysfb_init() comes at last, it
registers "efi-framebuffer" and then causes an error message "efifb: a
framebuffer is already registered". Make sysfb_init() to be subsys_
initcall_sync() can avoid this. And Javier Martinez Canillas is trying
to make a more general solution in commit 873eb3b1 ("fbdev: Disable
sysfb device registration when removing conflicting FBs").

However, this patch still makes sense because it can make the screen
display as early as possible (We cannot move to subsys_initcall, since
sysfb_init() should be executed after PCI enumeration).

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220704011704.1418055-1-chenhuacai@loongson.cn