drm: Small optimization to intel_dp_mst_atomic_master_trans_check

If we want to return from for_each_intel_connector_iter(), one
way is calling drm_connector_list_iter_end() before returning
to avoid memleak. The other way is just breaking from the bracket
and then returning after the outside drm_connector_list_iter_end().
Obviously, the second way makes code smaller and more clear.
Apply it to the function intel_dp_mst_atomic_master_trans_check().

Signed-off-by: He Ying <heying24@huawei.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211022022243.138860-1-heying24@huawei.com