drm/amd/display: Check Vactive for VRR active for FPO + Vactive

[Description]
- For FPO + Vactive cases, we rely on the Vactive display to be at
  it's nominal refresh rate because the Vactive pipe may not necessarily
  assert P-State allow while it's in VBLANK
- For cases where the Vactive display has a stretched VBLANK due to
  VRR, we could underflow when trying to complete an FPO + Vactive
  MCLK switch because the FPO display has limited VBLANK time in
  waiting for the Vactive display to assert P-State allow naturally
- Block FPO + Vactive if the Vactive display has VRR active (variable
  or fixed)

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Alex Hung <alex.hung@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>