package/x11r7/xdriver_xf86-video-mach64: don't override XDRIVER_XF86_VIDEO_MACH64_CONF_OPTS

XDRIVER_XF86_VIDEO_MACH64_CONF_OPTS is wrongly overridden in a
conditional since commit cf26ae7dd14f35dd917811f5a23c54a19ea6ed07

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/x11r7/xdriver_xf86-video-mach64/xdriver_xf86-video-mach64.mk

@@ -13,7 +13,7 @@ XDRIVER_XF86_VIDEO_MACH64_AUTORECONF = YES
 XDRIVER_XF86_VIDEO_MACH64_DEPENDENCIES = xserver_xorg-server xorgproto
 
 ifeq ($(BR2_PACKAGE_MESA3D_DRI_DRIVER),)
-XDRIVER_XF86_VIDEO_MACH64_CONF_OPTS = --disable-dri
+XDRIVER_XF86_VIDEO_MACH64_CONF_OPTS += --disable-dri
 endif
 
 $(eval $(autotools-package))