drm/amd/display: Add support for zstate during extended vblank

[why]
When we enter FREESYNC_STATE_VIDEO, we want to use the extra vblank
portion to enter zstate if possible.

[how]
When we enter freesync, a full update is triggered and the new vtotal
with extra lines is passed to dml in a stream update. The time gained
from extra vblank lines is calculated in microseconds. We allow zstate
entry if the time gained is greater than 5 ms, which is the current
policy. Furthermore, an optimized value for min_dst_y_next_start is
calculated and written to its register. When exiting freesync, another
full update is triggered and default values are restored.

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