meson.build: KMS support also depends on dri2

Kernel modesettings support also depends on dri2, see
./hw/xfree86/drivers/modesetting/meson.build

So update meson.build to reflect the changes made in configure.ac by
commit 9c81b8f5

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
(cherry picked from commit 5d73a8b5)