agp: define proper stubs for empty helpers

The empty unmap_page_from_agp() macro causes a warning when
building with 'make W=1' on a couple of architectures:

drivers/char/agp/generic.c: In function 'agp_generic_destroy_page':
drivers/char/agp/generic.c:1265:28: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
 1265 |   unmap_page_from_agp(page);

Change the definitions to a 'do { } while (0)' construct to
make these more reliable.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de> # parisc
Signed-off-by: Helge Deller <deller@gmx.de>