package/x11r7/xdriver_xf86-video-tdfx: don't override XDRIVER_XF86_VIDEO_TDFX_CONF_OPTS

XDRIVER_XF86_VIDEO_TDFX_CONF_OPTS is wrongly overridden in a conditional
since commit 7614ca037b2f82ac8e7cca1db3bf173d3655e0e2

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

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

@@ -16,7 +16,7 @@ XDRIVER_XF86_VIDEO_TDFX_DEPENDENCIES = \
 	xserver_xorg-server
 
 ifeq ($(BR2_PACKAGE_MESA3D_DRI_DRIVER),)
-XDRIVER_XF86_VIDEO_TDFX_CONF_OPTS = --disable-dri
+XDRIVER_XF86_VIDEO_TDFX_CONF_OPTS += --disable-dri
 endif
 
 $(eval $(autotools-package))