[Why]
Current periodic interrupt start point calc in optc
is not clear.

[How]
1. DM convert delta time to lines number and dc will calculate the
   start position as per lines number and interrupt type.
2. hwss calculates the start point as per line offset.
3. optc programs vertical interrupts register as per start point
   and interrupt source.
Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>