[PATCH v2] 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: 68f5d3f3b654 ("um: add PCI over virtio emulation driver")
Signed-off-by: Randy Dunlap <rdunlap at infradead.org>
Cc: David Airlie <airlied at linux.ie>
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: dri-devel at lists.freedesktop.org
Cc: Jeff Dike <jdike at addtoit.com>
Cc: Richard Weinberger <richard at nod.at>
Cc: Anton Ivanov <anton.ivanov at cambridgegreys.com>
Cc: linux-um at lists.infradead.org
---
v2: update Fixes: tag (thanks, Johannes)

 drivers/gpu/drm/r128/ati_pcigart.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- linux-next-20211007.orig/drivers/gpu/drm/r128/ati_pcigart.c
+++ linux-next-20211007/drivers/gpu/drm/r128/ati_pcigart.c
@@ -215,7 +215,7 @@ 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();