drm/amd/display: Disable eDP with a proper sequence.

Proper sequence should be:
disable backlight
dp blank
disable output
edp power off

In enable accelatate mode, all the encoder and controller
are disabled, so move disable eDP to the function is the
easiest way to implement.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>