drm/amd/display: Reduce SubVP + DRR stretch margin

[Description]
- Having excessively large margin causes failure in the static
  schedulability check in some cases for SubVP + DRR
- 100us of DRR margin is sufficient based on a weeks worth of
  stress testing on different display configs

Reviewed-by: Michael Strauss <Michael.Strauss@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>