drm/amd/display: Use absolute time stamp to follow the eDP T7 spec requirement

[Why]:
According to eDP spec, max T7 delay should be 50 ms. Current code uses 300
retry counters may not be accurate enough for different panels.

[How]:
Use absolute time stamp to achive accurate delay.

Signed-off-by: Dale Zhao <dale.zhao@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>