drm/amd/display: dc/bios: add support for DCE6

[Why]
command_table_helper.c requires changes for DCE6 support

[How]
DCE6 targets added replicating and adapting the existing DCE8 implementation.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>