drm/amd: Add debug mask for subviewport mclk switch

[Why&How]
Expose a new dc debug mask enum to force a subviewport memory clock switch
to facilitate easy testing.

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