drm/amd/display: Add margin for max vblank time for SubVP + DRR

[Description]
- Incorporate FW delays as port of max VTOTAL calculated for
  SubVP + DRR cases (since it is part of the microschedule).
- Also add margin for the max VTOTAL possible for SubVP + DRR cases.
- Due to rounding errors in FW (integer arithmetic), the microschedule
  calculation can get pushed to the next frame (incorrectly) in cases
  where we use the max VTOTAL possible to complete the MCLK switch.
- When the rounding error occurs, we are only off by 1-2 lines,
  use 40us margin which is working consistently.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@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>