drm/amd/display: Read down-spread percentage from lut to adjust dprefclk.

[Why]
Panels show corruption with high refresh rate timings when
ss is enabled.

[How]
Read down-spread percentage from lut to adjust dprefclk.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Martin Tsai <martin.tsai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>