ia64: agp: Replace empty define with do while

It's dangerous to use empty code define.
Furthermore it lead to the following warning:
drivers/char/agp/generic.c: In function « agp_generic_destroy_page »:
drivers/char/agp/generic.c:1266:28: attention : suggest braces around empty body in an « if » statement [-Wempty-body]

So let's replace emptyness by "do {} while(0)"
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1574324085-4338-6-git-send-email-clabbe@baylibre.com

arch/ia64/include/asm/agp.h

@@ -14,8 +14,8 @@
  * in coherent mode, which lets us map the AGP memory as normal (write-back) memory
  * (unlike x86, where it gets mapped "write-coalescing").
  */
-#define map_page_into_agp(page)		/* nothing */
-#define unmap_page_from_agp(page)	/* nothing */
+#define map_page_into_agp(page)		do { } while (0)
+#define unmap_page_from_agp(page)	do { } while (0)
 #define flush_agp_cache()		mb()
 
 /* GATT allocation. Returns/accepts GATT kernel virtual address. */