mesa: use _mesa_has_compute_shaders instead of extension check

This was really the original purpose, for enabling the path for
ES3.1 tests without the extension being set. Set also fallthrough
comment for Coverity (caught by Matt).

v2: .. and test the right way, not wrong one (Ilia Mirkin)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/mesa/main/shader_query.cpp

@@ -28,6 +28,7 @@
  * \author Ian Romanick <ian.d.romanick@intel.com>
  */
 
+#include "main/context.h"
 #include "main/core.h"
 #include "glsl_symbol_table.h"
 #include "ir.h"
@@ -986,8 +987,9 @@ _mesa_program_resource_prop(struct gl_shader_program *shProg,
    case GL_ACTIVE_VARIABLES:
       return get_buffer_property(shProg, res, prop, val, caller);
    case GL_REFERENCED_BY_COMPUTE_SHADER:
-      if (!ctx->Extensions.ARB_compute_shader)
+      if (!_mesa_has_compute_shaders(ctx))
          goto invalid_enum;
+      /* fallthrough */
    case GL_REFERENCED_BY_VERTEX_SHADER:
    case GL_REFERENCED_BY_GEOMETRY_SHADER:
    case GL_REFERENCED_BY_FRAGMENT_SHADER: