drm/amd/display: Add 16 lines margin for SubVP

[Description]
SUBVP_START_LINE must be aligned to 2 swaths, so add
16 lines of margin so the start line can be adjusted by
up to 16 lines for alignment purposes in FW.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>