drm/amd/display: Reset scrambling on Test Pattern

[Why]
Programming is missing the sequence where for eDP the scrambling is
reset when testing for eye diagram test pattern.

[How]
Include the required register in the definition

Signed-off-by: Chris Park <Chris.Park@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>