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>

src/gallium/frontends/dri/loader_dri3_helper.c

@@ -1611,7 +1611,7 @@ dri3_alloc_render_buffer(struct loader_dri3_drawable *draw, unsigned int fourcc,
       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]);

src/gallium/frontends/dri/loader_dri3_helper.h

@@ -71,7 +71,6 @@ struct loader_dri3_buffer {
    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;