drm/amd/display: DCN2.1 Disable 48MHz Powerdown Debug Option

[WHY & HOW]
Currently disable 48mhz debug option only disables on boot.
Need to put option check in update_clocks as well to make it
affect more areas.

Signed-off-by: Sung Lee <sung.lee@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>