modesetting: fix shared pixmap dmabuf fd leakage

After the dmabuf fd exported by another screen is imported to a pixmap,
the pixmap holds a reference for the buffer, thus the FD itself finished
its job and needs to be closed to prevent a stale reference to the
buffer.

Signed-off-by: Icenowy Zheng <uwu@icenowy.me>

hw/xfree86/drivers/modesetting/driver.c

@@ -1876,6 +1876,7 @@ msSetSharedPixmapBacking(PixmapPtr ppix, void *fd_handle)
                                              ppix->devKind,
                                              ppix->drawable.depth,
                                              ppix->drawable.bitsPerPixel);
+        close(ihandle);
     } else {
         int size = ppix->devKind * ppix->drawable.height;
         ret = drmmode_SetSlaveBO(ppix, &ms->drmmode, ihandle, ppix->devKind, size);