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>