drm/amd: Duplicate DC_FEATURE_MASK and DC_DEBUG_MASK enum values into kdoc

[Why]
When kernel documentation is generated the enum values themselves don't
end up in the documentation.  This makes browsing them in HTML a lot
less useful.

[How]
Copy DC_DEBUG_MASK and DC_FEATURE_MASK enum values into matching kdoc
comments.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>