x86/fbdev: Include asm/fb.h as needed

fb_is_primary_device() is defined as a global function on x86, unlike
the others that have an inline version. The file that defines is
however needs to include the declaration to avoid a warning:

arch/x86/video/fbdev.c:14:5: error: no previous prototype for 'fb_is_primary_device' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://lore.kernel.org/all/20230516193549.544673-15-arnd%40kernel.org