drm/amd/display: Optimize vstartup position for AS-SDP

[Why]
In current design, the vstartup position is adjusted to
vblank start position when AS-SDP is enabled.
However when the vblank length is too big, it may over
vstartup boundary.

[How]
To adjust vstartup position to 1 line before vsync position.

Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Signed-off-by: Robin Chen <robin.chen@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>