drm/amd/display: Change DCN2 vupdate start programming

[Why]
In order to ensure that incoming flips are latched and
complete immediately, we need to program the vupdate
interrupt to come during the back porch of each frame.

[How]
Program the vupdate start_line to be in the back porch
like it's done for DCN1.

Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>