drm/amd/display: Check and log for function error codes

[WHAT & HOW]
dm_dmub_hw_init and drm_dp_mst_topology_mgr_set_mst can return error
codes and errors should be reported.

This fixes 2 CHECKED_RETURN issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>