radeon/dri2: if the depth buffer is 16-bit force cpp == 2

This is because the DDX always allocates using the drawable sizes.
which gives me twice the depth buffer I asked for, dumb.

src/mesa/drivers/dri/radeon/radeon_common_context.c

@@ -508,8 +508,11 @@ radeon_update_renderbuffers(__DRIcontext *context, __DRIdrawable *drawable)
 			}
 		}
 
-		if (buffers[i].attachment == __DRI_BUFFER_DEPTH)
+		if (buffers[i].attachment == __DRI_BUFFER_DEPTH) {
+			if (draw->base.Visual.depthBits == 16)
+				rb->cpp = 2;
 			depth_bo = bo;
+		}
 
 		radeon_renderbuffer_set_bo(rb, bo);
 		radeon_bo_unref(bo);