turnip: Add kgsl backend

Lacking a bit around fences and wsi integration, but there's enough
here to actually drive the GPU.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4479>
diff --git a/meson.build b/meson.build
index fa178c4..3b7e115 100644
--- a/meson.build
+++ b/meson.build
@@ -263,6 +263,7 @@
 with_amd_vk = _vulkan_drivers.contains('amd')
 with_freedreno_vk = _vulkan_drivers.contains('freedreno')
 with_swrast_vk = _vulkan_drivers.contains('swrast')
+with_freedreno_kgsl = get_option('freedreno-kgsl')
 with_any_vk = _vulkan_drivers.length() != 0
 
 if with_swrast_vk and not with_gallium_softpipe