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();
^~~~~~