drm/amd/display: change number of cursor policy for dml calculation.

[Why]
vstartup calculation is incorrect due to use 2 number of cursors and
result in an underflow when playing video in full screen mode and
combines graphic plane and video plane.

[How]
Apply new policy for dml calculation.
1 cursor for graphic plane, 0 cursor for video plane.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>