drm/amd/display: update extended blank for dcn314 onwards

[Why]
Flickering and underflow was observed when testing extended
blank on dcn314.

[What]
Vstartup is contrainted by vblank_nom, so adjusting it to include
non-adjusted vtotal in its calculation during freesync video
means that Vstartup is not changed when vtotal changes.
This fixed the flickering + underflow.

dc_extended_blank_supported function was removed
because extended blank is only relevant to when
zstate is supported. The increased vtotal during
freesync can be passed to dml regardless of whether
extended blank is supported or not, so this function is
not needed.

Updates were made recently in dml to the calculation of
min_dst_y_next_start. Dml input for dcn314 will now
always use the newer calculation for min_dst_y_next_start.
Dml input for older dcn versions remains untouched.

The variable optimized_min_dst_y_next_start
is replaced everywhere with min_dst_y_next_start,
and the updated dml allows min_dst_y_next_start to
increase to an optimized value during freesync video,
then return to default when freesync is disengaged.

Also removed registry key for controlling
extended blank feature.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Gabe Teeger <gabe.teeger@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>