mesa3d: use positive logic for XA enabling

Thanks to commit 2dde4008
("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>