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

[Why & How]
Revert commit 6917b0b7 ("drm/amd/display: Read down-spread percentage from lut to adjust dprefclk.")
This change was causing 240hz display to not light up after s0i3.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Leo Chen <sancchen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>