gallium: Require LLVM >= 3.9

To go any further than this would be to break the current version of
Android.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
diff --git a/meson.build b/meson.build
index 400427e..b8c1999 100644
--- a/meson.build
+++ b/meson.build
@@ -1261,10 +1261,8 @@
   _llvm_version = '>= 8.0.0'
 elif with_gallium_swr
   _llvm_version = '>= 6.0.0'
-elif with_gallium_opencl or with_gallium_r600
-  _llvm_version = '>= 3.9.0'
 else
-  _llvm_version = '>= 3.8.0'
+  _llvm_version = '>= 3.9.0'
 endif
 
 _shared_llvm = get_option('shared-llvm')