drm/amd/display: Avoid trying AUX transactions on disconnected ports

[Why & How]
Observe that we try to access DPCD 0x600h of disconnected DP ports.
In order not to wasting time on retrying these ports, call
dpcd_write_rx_power_ctrl() after checking its connection status.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>