parisc/stifb: Fix fb_is_primary_device() only available with CONFIG_FB_STI

Fix this build error noticed by the kernel test robot:

drivers/video/console/sticore.c:1132:5: error: redefinition of 'fb_is_primary_device'
 arch/parisc/include/asm/fb.h:18:19: note: previous definition of 'fb_is_primary_device'

Signed-off-by: Helge Deller <deller@gmx.de>
Reported-by: kernel test robot <lkp@intel.com>
Cc: stable@vger.kernel.org   # v5.10+