mesa3d: use positive logic for XA enabling

Thanks to commit 2dde40081c49852bac0ee9c723911761bd5fc91e
("package/mesa3d: Refactor XA support to prepare support for ATI
drivers"), we now have the blind option BR2_PACKAGE_MESA3D_NEEDS_XA
that indicates whether XA support is needed. This allows to switch to
use positive logic in the test for XA in mesa3d.mk, which this commit
does.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/mesa3d/mesa3d.mk

@@ -36,7 +36,7 @@ MESA3D_DEPENDENCIES += \
 	libxcb
 MESA3D_CONF_OPTS += --enable-glx
 # quote from mesa3d configure "Building xa requires at least one non swrast gallium driver."
-ifneq ($(BR2_PACKAGE_MESA3D_NEEDS_XA),)
+ifeq ($(BR2_PACKAGE_MESA3D_NEEDS_XA),y)
 MESA3D_CONF_OPTS += --enable-xa
 else
 MESA3D_CONF_OPTS += --disable-xa