drm/amd/display: Don't update drm connector when read local EDID

[Why]
Currently, when we're tentatively detecting link configuration under mst
case, we will wrongly notify a none existing stream sink to drm layer.

It's due to we'll call amdgpu_dm_update_connector_after_detect() within
dm_helpers_read_local_edid() to update connector when we're still under
tentative detection procedure.

[How]
dm_helpers_read_local_edid() was designed to do "read" only. Don't
update drm connector status when we're still under detection process.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>