drm/amd/display: Add and use new dm_prepare_suspend() callback

[Why]
The displays currently don't get turned off until after other IP blocks
have been suspended.  However turning off the displays first gives a
very visible response that the system is on it's way down.

[How]
Turn off displays in a prepare_suspend() callback instead when possible.
This will help for suspend and hibernate sequences.
The shutdown sequence however will not call prepare() so check whether
the state has been already saved to decide what to do.

Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>