dri3: Set stride and size for old clients

For old clients using the fd_from_pixmap entrypoint, make sure we set
stride and size correctly.

Noticed by inspection.

Signed-off-by: Daniel Stone <daniels@collabora.com>

dri3/dri3_screen.c

@@ -161,6 +161,8 @@ dri3_fd_from_pixmap(PixmapPtr pixmap, CARD16 *stride, CARD32 *size)
         return -1;
     }
 
+    *stride = strides[0];
+    *size = size[0];
     return fds[0];
 }