drm/msm/dp: fix atomic_best_enc select for DP MST connectors

Use already selected encoder for mst connector in case
atomic_best_encoder op is called multiple times during
drm atomic commit cycle.

CRs-Fixed: 2192818
Change-Id: I7f9093c8edd101221025acc4e395d1a1478e2070
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>