meson: remove source_root() call in nir compiler path

source_root function is deprecated in Meson version 0.56.0, so let's use
instead a current_source_dir() function, available in all Meson
versions. This also allows to deduplicate some code by declaring
commonly used string at the top meson.build file.

Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17974>

src/intel/compiler/meson.build

@@ -149,8 +149,7 @@ brw_nir_trig = custom_target(
   input : 'brw_nir_trig_workarounds.py',
   output : 'brw_nir_trig_workarounds.c',
   command : [
-    prog_python, '@INPUT@',
-    '-p', join_paths(meson.source_root(), 'src/compiler/nir/'),
+    prog_python, '@INPUT@', '-p', dir_compiler_nir,
   ],
   depend_files : nir_algebraic_depends,
   capture : true,