dri3: Fix pixmap buf_id computation

Looks like some kind of rebase damage to me...

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
diff --git a/src/glx/dri3_glx.c b/src/glx/dri3_glx.c
index 61cb2de..0c8e064 100644
--- a/src/glx/dri3_glx.c
+++ b/src/glx/dri3_glx.c
@@ -945,7 +945,7 @@
                        void *loaderPrivate)
 {
    struct dri3_drawable                 *pdraw = loaderPrivate;
-   int                                  buf_id = buffer_type == dri3_pixmap_buf_id(buffer_type);
+   int                                  buf_id = dri3_pixmap_buf_id(buffer_type);
    struct dri3_buffer                   *buffer = pdraw->buffers[buf_id];
    Pixmap                               pixmap;
    xcb_dri3_buffer_from_pixmap_cookie_t bp_cookie;