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>

drivers/gpu/drm/amd/display/dc/dcn21/dcn21_resource.c

@@ -286,7 +286,7 @@ struct _vcs_dpi_soc_bounding_box_st dcn2_1_soc = {
 	.dram_channel_width_bytes = 4,
 	.fabric_datapath_to_dcn_data_return_bytes = 32,
 	.dcn_downspread_percent = 0.5,
-	.downspread_percent = 0.5,
+	.downspread_percent = 0.38,
 	.dram_page_open_time_ns = 50.0,
 	.dram_rw_turnaround_time_ns = 17.5,
 	.dram_return_buffer_per_channel_bytes = 8192,