drm/amd/display: eDP intermittent black screen during PnP

[Why]
We dont's turn off backlight before power off eDP (VDD),
which is a violation of eDP specs.

[How]
Power off eDP backlight before power off eDP

Signed-off-by: Peikang Zhang <peikang.zhang@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>