drm/amd/display: Added new DMUB boot option for power optimization

[Why]
During Z10, root clock gating and memory low power registers needs to
to be restored if optimization is enabled in driver.

[How]
Added new DMUB boot option for root clock gating and memory low power.

Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Jake Wang <haonan.wang2@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>