drm/amd/display: Extend soc BB capabilitiy

[why]
Some parts are consuming dangerously close to maximum number of states
supported when updating the BB (i.e. 8).

[how]
Change maximum stages from 9 to 20.
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Jun Lei <jun.lei@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>