drm/amd/display: Set max vratio for prefetch to 7.9 for YUV420 MPO

[Description]
- Single 4K60 playing YUV420 MPO video blocks P-State because
  the required VRatio for prefetch is too high (luma plane for
  YUV420 is 1bpe, so swath height is 16 and prefetch requires
  more lines)
- Allow max vratio per plane to be 7.9 for single display YUV420
  MPO video cases
- Ensure that global vratio prefetch (i.e. total prefetch BW vs.
  total active bandwidth) does not excited 4.0

Reviewed-by: Nevenko Stupar <Nevenko.Stupar@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>