[WHY]
Memory clock switching has great potential for power savings.

[HOW]
The driver code was modified to notify the DMCUB firmware that it should
stretch the vertical blank of frames when a memory clock switch is about
to start so that no blackouts happen on the screen due to unavailability
of the frame buffer.
The driver logic to determine when such firmware assisted strategy can
be initiated is also implemented and consists on checking prerequisites
of the feature.
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Felipe Clark <felipe.clark@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>