drm/amd/display: Add handling for DC power mode

[Why]
Future implementations will require a distinction between AC power and
DC power (wall power and battery power, respectively). To accomplish this,
adding a power mode parameter to certain dc interfaces, and adding a
separate DML2 instance for DC mode validation. Default behaviour unchanged.

Reviewed-by: Jun Lei <jun.lei@amd.com>
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>