drm/amd/display: Use int for calculating vline start

We are not sure these calculations will never need negative numbers. Use
signed integers and warn and cap at 0 if this ever happens.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>