drm/amd/display: define abm debug interface

[Why & How]
Define debug interface to dmub for reading back abm data.

Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Josip Pavic <josip.pavic@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>