drm/amd/display: add delay between panel pwr off to on.

As per eDP 1.4 spec, there must be at least 500ms delay
between eDP power off and on.
This change added time stamp when edp power off, which can
be used to calculate duration time when edp power on.
If duration less than 500ms, add a wait.

Signed-off-by: Charlene Liu <charlene.liu@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>