drm/vmwgfx: Fix regression caused by "drm/ttm: make ttm reservation calls behave like reservation calls"

The call to ttm_eu_backoff_reservation() as part of an error path would cause
a lock imbalance if the reservation ticket was not initialized. This error is
easily triggered from user-space by submitting a bogus command stream.
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Cc: stable@vger.kernel.org
Cc: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>