[Why]
The eDP retrain will cause the DPCD 300 to be reset to default.
And cause the brightness can't be set correctly.

[How]
delete the call to edp panel power control in both
enable_link_output/disable_link_output entirely and
only call edp panel control in enable_link_dp and 
disable_link_dp once.
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Jingwen Zhu <Jingwen.Zhu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>