drm/amd/display: cache pwl params and scl_data to avoid extra programming

This saves us about 5000 reg writes per full update. This translates to about
40000 writes over the course of single eDP bootup.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@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>