gallium/dri3: Drop loader_dri3_buffer::size

It's never set outside of the automatic zero from calloc().  Having it
in there is decpetive.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36014>
diff --git a/src/gallium/frontends/dri/loader_dri3_helper.c b/src/gallium/frontends/dri/loader_dri3_helper.c
index 37970f4..7b0c4ea 100644
--- a/src/gallium/frontends/dri/loader_dri3_helper.c
+++ b/src/gallium/frontends/dri/loader_dri3_helper.c
@@ -1611,7 +1611,7 @@
       cookie_pix = xcb_dri3_pixmap_from_buffer_checked(draw->conn,
                                                        pixmap,
                                                        draw->drawable,
-                                                       buffer->size,
+                                                       0,
                                                        width, height, buffer->strides[0],
                                                        depth, buffer->cpp * 8,
                                                        buffer_fds[0]);
diff --git a/src/gallium/frontends/dri/loader_dri3_helper.h b/src/gallium/frontends/dri/loader_dri3_helper.h
index 4da19d7..26f138d 100644
--- a/src/gallium/frontends/dri/loader_dri3_helper.h
+++ b/src/gallium/frontends/dri/loader_dri3_helper.h
@@ -71,7 +71,6 @@
    bool         reallocate;     /* Buffer should be reallocated and not reused */
 
    uint32_t     num_planes;
-   uint32_t     size;
    int          strides[4];
    int          offsets[4];
    uint64_t     modifier;