arch/parisc: Detect primary video device from device instance

Update fb_is_primary device() on parisc to detect the primary display
device from the Linux device instance. Aligns the code with the other
architectures. A later patch will remove the fbdev dependency from the
function's interface.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>