drm/amd/display: changed max_downscale_src_width to 4096.

[Why]
MPO isn't enabled on some 4k videos due to video source width is 4096
and the current limit is 3840.

[How]
Changed the limit to 4096.
Signed-off-by: Isabel Zhang <isabel.zhang@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -830,7 +830,7 @@ static const struct dc_debug_options debug_defaults_drv = {
 		.disable_dcc = DCC_ENABLE,
 		.vsr_support = true,
 		.performance_trace = false,
-		.max_downscale_src_width = 3840,
+		.max_downscale_src_width = 4096,
 		.disable_pplib_wm_range = false,
 		.scl_reset_length10 = true,
 		.sanity_checks = true,