drm/amd/display: Add a variable to update FCLK latency

[Description]
Add a variable to update FCLK latency

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>