drm/amd/display: Update downspread percent to match spreadsheet for DCN2.1

[WHY]
The downspread percentage was copied over from a previous version
of the display_mode_lib spreadsheet. This value has been updated,
and the previous value is too high to allow for such modes as
4K120hz. The new value is sufficient for such modes.

[HOW]
Update the value in dcn21_resource to match the spreadsheet.

Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>