drm/amd/display: FreeSync not active near lower bound of non-LFC monitor range

[Why]
On narrow range monitors without LFC, a margin prevents good utilization
of the available range.

[How]
Decrease the margin for exiting fixed mode and fix the frame counter to
reset if a non-consecutive render is found.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>