weston: Re-order gbm destruction at DRM-backend tear down

weston: Re-order gbm destruction at DRM-backend tear down

Fixes an annoying segfault seen on rpi4 when launching weston during
boot especially using systemd service or sysvinit script, if weston is
launched manually on VT, it works fine. This backport specifically
addresses the issue described above.

For more details see [1]

[1] https://gitlab.freedesktop.org/wayland/weston/-/issues/314

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>