arch/x86: Implement <asm/fb.h> with generic helpers

Include <asm-generic/fb.h> and set the required preprocessor tokens
correctly. x86 now implements its own set of fb helpers, but still
follows the overall pattern of the other <asm/fb.h> files.

v3:
	* clarified commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417125651.25126-20-tzimmermann@suse.de