drm/amd/display: Add FPO + VActive support

[Description]
- When determining FPO support, include FPO + VActive support
- Support FPO + VActive if one display meets regular requirements
  for FPO and the second display is able to switch in VACTIVE with
  a given amount of margin

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>