[PATCH] drm/r128: fix build for UML

[Randy Dunlap]

Fix a build error on CONFIG_UML, which does not support (provide)
wbinvd(). UML can use the generic mb() instead.

../drivers/gpu/drm/r128/ati_pcigart.c: In function ‘drm_ati_pcigart_init’:
../drivers/gpu/drm/r128/ati_pcigart.c:218:2: error: implicit declaration of 
function ‘wbinvd’ [-Werror=implicit-function-declaration]
  wbinvd();
  ^~~~~~

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") # pre-git
Signed-off-by: Randy Dunlap <rdun...@infradead.org>
Cc: David Airlie <airl...@linux.ie>
Cc: Daniel Vetter <dan...@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: Jeff Dike <jd...@addtoit.com>
Cc: Richard Weinberger <rich...@nod.at>
Cc: Anton Ivanov <anton.iva...@cambridgegreys.com>
Cc: linux...@lists.infradead.org
---
 drivers/gpu/drm/r128/ati_pcigart.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- linux-next-20210901.orig/drivers/gpu/drm/r128/ati_pcigart.c
+++ linux-next-20210901/drivers/gpu/drm/r128/ati_pcigart.c
@@ -214,7 +214,7 @@ int drm_ati_pcigart_init(struct drm_devi
        }
        ret = 0;
 
-#if defined(__i386__) || defined(__x86_64__)
+#if (defined(__i386__) || defined(__x86_64__)) && !defined(CONFIG_UML)
        wbinvd();
 #else
        mb();