drm/amd/display: Freesync does not engage on some displays

[Why]
Current render margin time is not sufficient to compute exit frame
time for most monitors.

[How]
Declared  render margin in FPS to compute a exit frame rate that is
4 FPS above the minimum FPS required to engage FreeSync.
 Also did code clean-up to remove redundancies.

Signed-off-by: Harmanprit Tatla <htatla@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>