[intel] Move bufmgr back to context instead of screen, fixing glthreads.

Putting the bufmgr in the screen is not thread-safe since the emit_reloc
changes.  It also led to a significant performance hit from pthread usage
for the attempted thread-safety (up to 12% of a cpu spent on refcounting
protection in single-threaded 965).  The motivation had been to allow
multi-context bufmgr sharing in classic mode, but it wasn't worth the cost.

src/mesa/drivers/dri/intel/intel_fbo.c

@@ -282,7 +282,7 @@ intel_alloc_renderbuffer_storage(GLcontext * ctx, struct gl_renderbuffer *rb,
       DBG("Allocating %d x %d Intel RBO (pitch %d)\n", width,
 	  height, pitch);
 
-      irb->region = intel_region_alloc(intel->intelScreen, cpp, pitch, height);
+      irb->region = intel_region_alloc(intel, cpp, pitch, height);
       if (!irb->region)
          return GL_FALSE;       /* out of memory? */