drm/amd/display: Add DCHUBBUB callback to report MALL status

drm/amd/display: Add DCHUBBUB callback to report MALL status

[Why&How]
For enabling automated testing, add a hook to DCHUBBUB interface so that
mall status can be queried by userspace through debugfs. This removes
dependence on requiring a userspace tool like UMR for querying status
for MALL static screen IGT test.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>