virgl: close drm fd when destroying virgl screen.

This fd was create in virgl_drm_screen_create and should be closed
in virgl_drm_screen_destroy.

Signed-off-by: Lepton Wu <lepton@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>

src/gallium/winsys/virgl/drm/virgl_drm_winsys.c

@@ -972,6 +972,7 @@ virgl_drm_screen_destroy(struct pipe_screen *pscreen)
    if (destroy) {
       int fd = virgl_drm_winsys(screen->vws)->fd;
       util_hash_table_remove(fd_tab, intptr_to_pointer(fd));
+      close(fd);
    }
    mtx_unlock(&virgl_screen_mutex);