drm/amd/display: Remove v_startup workaround for dcn3+

[Why]
Calls to dcn20_adjust_freesync_v_startup are no longer
needed as of dcn3+ and can cause underflow in some cases

[How]
Move calls to dcn20_adjust_freesync_v_startup up into
validate_bandwidth for dcn2.x

Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Daniel Miess <daniel.miess@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>