drm/nouveau/kms: Don't clear DP_MST_CTRL DPCD in nv50_mstm_new()

Since commit fa3cdf8d ("drm/nouveau: Reset MST branching unit before
enabling") we've been clearing DP_MST_CTRL before we start enabling MST.
Since then clearing DP_MST_CTRL in nv50_mstm_new() has been unnecessary
and redundant, so let's remove it.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-6-lyude@redhat.com