drm/amd/display: Add a hysteresis to BTR frame multiplier

[Why]
Flickering is observed on some displays when the number of inserted BTR
frames changes frequently.

[How]
Add in a margin of drift to prevent the inserted number of frames from
jumping around too frequently.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>