drm/amd/pm: add deep sleep control for uclk and fclk

These are supported by Sienna Cichlid and should be
taken into consideration during DS control.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>