[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_blit.c

@@ -459,7 +459,7 @@ intelClearWithBlit(GLcontext * ctx, GLbitfield mask)
                struct intel_region *irb_region =
 		  intel_get_rb_region(fb, buf);
                dri_bo *write_buffer =
-                  intel_region_buffer(intel->intelScreen, irb_region,
+                  intel_region_buffer(intel, irb_region,
                                       all ? INTEL_WRITE_FULL :
                                       INTEL_WRITE_PART);