drm/amd/display: allow dscclk disable

[why]
when dscclk rcg disabled from usr reg option,
dsc clock will remain enabled because driver was doing two things
both dscclk and dsc rcg in the same routine.

Reviewed-by: Hansen Dsouza <hansen.dsouza@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>