drm/i915/dg2: Update to bigjoiner path

In verify_mpllb_state() encoder is retrieved from best_encoder
of connector_state. As there will be only one connector_state
for bigjoiner and checking encoder may not be needed for
bigjoiner-slave. This code path related to mpll is done on dg2
and need this fix to avoid null pointer dereference issue.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723174239.1551352-30-matthew.d.roper@intel.com