amd: remove support for LLVM 5.0

Users are encouraged to switch to LLVM 6.0 released in March 2018.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
diff --git a/meson.build b/meson.build
index 9a15c8a..cbaee5c 100644
--- a/meson.build
+++ b/meson.build
@@ -1151,7 +1151,9 @@
   llvm_optional_modules += ['coroutines', 'opencl']
 endif
 
-if with_amd_vk or with_gallium_radeonsi or with_gallium_swr
+if with_amd_vk or with_gallium_radeonsi
+  _llvm_version = '>= 6.0.0'
+elif with_gallium_swr
   _llvm_version = '>= 5.0.0'
 elif with_gallium_opencl or with_gallium_r600
   _llvm_version = '>= 3.9.0'