The next patch adds sending a modeset-retry uevent after a link training
failure to all MST connectors on link. This requires the atomic state,
so pass it to intel_dp_start_link_train(). In case of SST where
retraining still happens by calling this function directly instead of a
modeset commit the atomic state is not available and NULL is passed
instead. This is ok, since in this case the encoder's only DP connector
is available from intel_dp->attached_connector not requiring the atomic
state.

v2: Add NOTE that the atomic state may not be valid for SST links and
    assert that it's valid for MST links. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610164933.2947366-10-imre.deak@intel.com