drm/amd/display: Restrict multi-disp support for in-game FAMS

[HOW&WHY]
In multi-monitor cases the VBLANK stretch that is required to align both
monitors may be so large that it may create issues for gaming performance.

Use debug value to restrict in-game FAMS support for multi-disp use case.

Reviewed-by: Harry Vanzylldejong <harry.vanzylldejong@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Iswara Nagulendran <iswara.nagulendran@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>