ac,radv,radeonsi: remove LLVM 7 support

Now that LLVM 9 will be released soon, we will only support
LLVM 8, 9 and master (10).

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
diff --git a/meson.build b/meson.build
index bf61511..501957e 100644
--- a/meson.build
+++ b/meson.build
@@ -1243,7 +1243,7 @@
 endif
 
 if with_amd_vk or with_gallium_radeonsi
-  _llvm_version = '>= 7.0.0'
+  _llvm_version = '>= 8.0.0'
 elif with_gallium_swr
   _llvm_version = '>= 6.0.0'
 elif with_gallium_opencl or with_gallium_r600