meson: Include iris in default gallium-drivers for x86/x86_64

We build i965 by default on x86/x86_64 platforms; let's build iris too.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

meson.build

@@ -166,7 +166,8 @@ if gallium_drivers.contains('auto')
     # TODO: PPC, Sparc
     if ['x86', 'x86_64'].contains(host_machine.cpu_family())
       gallium_drivers = [
-        'r300', 'r600', 'radeonsi', 'nouveau', 'virgl', 'svga', 'swrast'
+        'r300', 'r600', 'radeonsi', 'nouveau', 'virgl', 'svga', 'swrast',
+        'iris'
       ]
     elif ['arm', 'aarch64'].contains(host_machine.cpu_family())
       gallium_drivers = [