meson: allow building dri driver without window system if osmesa is classic

This was already enabled for gallium based osmesa with gallium drivers
in 9d105818, so do the same for classic
driver with classic osmesa.

Fixes: cbbd5bb8
       ("meson: build classic osmesa")
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
(cherry picked from commit 431e9aba)