drm/amd/display: Disable physym clock

[How & Why]
Disable physym clock when it's not in use.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Eric Yang <Eric.Yang2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: David Galiffi <David.Galiffi@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>