drm/amd/display: Add debug option to force fclk request

[Why]
So that we can adjust fclk for debugging purposes.

[How]
Add option to force adjust fclk request to pplib.

Signed-off-by: Hugo Hu <hugo.hu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>