wayland: Fix lease termination in wp_lease_v1_destroy()

If a drm lease is destroyed, e.g. the app has unexpectedly terminated,
only the finished event will be sent. The leaseRevoked signal won't be
emitted so the drm backend can't clean up DrmOutput::m_lease. Since
m_lease can be a dangling pointer, the drm backend can crash in
DrmGpu::updateOutputs() when it tries to determine if m_lease is still
alive and was not terminated by closing the lease fd on the client side.