meson: Add panfrost to the --auto list

Look ma, we're a real driver now! I was waiting until Panfrost
stabilises a bit for this, but now that 19.2 is almost here, let's make
us official :)

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
diff --git a/meson.build b/meson.build
index 61fc864..a210b39 100644
--- a/meson.build
+++ b/meson.build
@@ -158,7 +158,7 @@
     elif ['arm', 'aarch64'].contains(host_machine.cpu_family())
       gallium_drivers = [
         'kmsro', 'v3d', 'vc4', 'freedreno', 'etnaviv', 'nouveau',
-        'tegra', 'virgl', 'lima', 'swrast'
+        'tegra', 'virgl', 'lima', 'panfrost', 'swrast'
       ]
     else
       error('Unknown architecture @0@. Please pass -Dgallium-drivers to set driver options. Patches gladly accepted to fix this.'.format(