drm/amd/display: Remove aconnector condition check for dpcd read

[Why]
Aconnector is not necessary to be NULL in order to read dpcd
successfully.

Actually if we rely on checking aconnector here, we won't be able
to turn off all displays before doing display detection. That will
cause some MST hubs not able to light up.

[How]
Remove aconnector check when turning off all displays at
hardware initialization stage.

Signed-off-by: Zhan Liu <zhan.liu@amd.com>
Reviewed-by: Joseph Gravenor <joseph.gravenor@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>