drm/amd/display: Clean up triple buffer enablement code

[WHY]
Triple buffer enablement currently does not work properly

[HOW]
Allow triple buffer enablement to happen properly on
fast updates

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Sung Lee <sunglee@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>