drm/amd/display: power down eDP if eDP not present

[Why]
When init, driver will power up eDP to check eDP
present or not. If eDP not present, driver doesn't
power off eDP VDD cause power consumption.

[How]
If eDP is not present when detection, power off eDP
VDD.

Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Paul Hsieh <Paul.Hsieh@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>