gallium: add flag PIPE_CONTEXT_PREFER_THREADED

State trackers can set this to tell the driver when u_threaded_context is
desirable.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>

src/gallium/include/pipe/p_defines.h

@@ -383,6 +383,14 @@ enum pipe_flush_flags
  */
 #define PIPE_CONTEXT_ROBUST_BUFFER_ACCESS (1 << 2)
 
+/**
+ * Prefer threaded pipe_context. It also implies that video codec functions
+ * will not be used. (they will be either no-ops or NULL when threading is
+ * enabled)
+ */
+#define PIPE_CONTEXT_PREFER_THREADED   (1 << 3)
+
+
 /**
  * Flags for pipe_context::memory_barrier.
  */

src/mesa/state_tracker/st_manager.c

@@ -647,7 +647,7 @@ st_api_create_context(struct st_api *stapi, struct st_manager *smapi,
    struct pipe_context *pipe;
    struct gl_config mode;
    gl_api api;
-   unsigned ctx_flags = 0;
+   unsigned ctx_flags = PIPE_CONTEXT_PREFER_THREADED;
 
    if (!(stapi->profile_mask & (1 << attribs->profile)))
       return NULL;