drm/atomic: Ensure that drm_connector_index is stable

I've totally forgotten that with DP MST connectors can now be
hotplugged. And failed to adapt Rob's drm_atomic_state code (which
predates connector hotplugging) to the new realities.

The first step is to make sure that the connector indices used to
access the arrays of pointers are stable. The connection mutex gives
us enough guarantees for that, which means we won't unecessarily block
on concurrent modesets or background probing.

So add a locking WARN_ON and shuffle the code slightly to make sure we
always hold the right lock.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>