drm/amd: Add a DC debug mask for DML2

[Why&How]
To enable testing/development of DML2, expose a new debug mask for future use.

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>