fix invalid usage of buffer_unreference()

Need to pass the address of the buffer ptr, not the buffer ptr.
Before, the region->buffer type was void * so it wasn't effectively
type-checked.  Changing the type to pipe_buffer_object* allows the
compiler to detect the error.  Fixing this solves a segfault.

src/mesa/pipe/i915simple/i915_regions.c

@@ -124,7 +124,7 @@ i915_region_release(struct pipe_context *pipe, struct pipe_region **region)
       assert((*region)->map_refcount == 0);
 
       i915->pipe.winsys->buffer_unreference( i915->pipe.winsys,
-					     (*region)->buffer );
+					     &((*region)->buffer) );
       free(*region);
    }
    *region = NULL;

src/mesa/pipe/softpipe/sp_region.c

@@ -119,7 +119,7 @@ sp_region_release(struct pipe_context *pipe, struct pipe_region **region)
       assert((*region)->map_refcount == 0);
 
       sp->pipe.winsys->buffer_unreference( sp->pipe.winsys,
-				      (*region)->buffer );
+                                           &((*region)->buffer) );
       free(*region);
    }
    *region = NULL;