drm/amd/display: Fix FreeSync active bit issue

[Why]
The FreeSync active bit unconditionally set in HDMI VSIF.

[How]
Set this bit to true when FAMS is enable on desktop.

Reviewed-by: Felipe Clark <felipe.clark@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>