arch/parisc: Implement fb_is_primary_device() under arch/parisc

Move PARISC's implementation of fb_is_primary_device() into the
architecture directory. This the place of the declaration and
where other architectures implement this function. No functional
changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417125651.25126-14-tzimmermann@suse.de