arch/sparc: Implement fb_is_primary_device() in source file

Other architectures implment fb_is_primary_device() in a source
file. Do the same on sparc. No functional changes, but allows to
remove several include statement from <asm/fb.h>.

v2:
	* don't include <asm/prom.h> in header file

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: "David S. Miller" <davem@davemloft.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417125651.25126-18-tzimmermann@suse.de