drm/amd/display: Fix prefetch vratio check

[Why & How]
- For prefetch max vratio check, use the calculated
  prefetch bandwidth from dml32_CalculatePrefetchSchedule
  instead of max prefetch bandwidth
- Also multiply prefetch bandwidth by VRatio since scaling
  is not considered one calculating require prefetch bw

Reviewed-by: Jun Lei <Jun.Lei@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>