drm/amd/display: Improve LFC behaviour

[Why]
There can be some unsynchronized frames when entering/exiting
LFC.  This may cause tearing or stuttering at such transitions.

[How]
Add a enter/exit margin to algorithm to smoothly transition into
and out of LFC without desynchronizing frames.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Reza Amini <Reza.Amini@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Sivapiriyan Kumarasamy <Sivapiriyan.Kumarasamy@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>