meson: Enable d3d12 in gallium_drivers by default on Windows

This is pretty much the only usable one that isn't
swrast for some people, eg. WoA + QCom.

Signed-off-by: Joshua Ashton <joshua@froggi.es>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27737>

meson.build

@@ -158,7 +158,7 @@ if gallium_drivers.contains('auto')
             host_machine.cpu_family()))
     endif
   elif ['windows'].contains(host_machine.system())
-    gallium_drivers = ['swrast', 'zink']
+    gallium_drivers = ['swrast', 'zink', 'd3d12']
   elif ['darwin', 'cygwin', 'haiku'].contains(host_machine.system())
     gallium_drivers = ['swrast']
   else